Fix case sensitivity fswatch and users (#64210)

tsc/internal/execute/watchmanager/watchmanager.go

@@ -293,14 +293,16 @@ func (wm *WatchManager) createDirWatches(updates []dirWatchUpdate) error {
 // already present in the set, or when it is contained within a recursive watch
 // directory already in the set.
 type DirWatchSet struct {
-	opts tspath.ComparePathsOptions
-	dirs map[string]bool
+	opts  tspath.ComparePathsOptions
+	dirs  map[string]bool
+	names map[string]string
 }
 
 func NewDirWatchSet(opts tspath.ComparePathsOptions) *DirWatchSet {
 	return &DirWatchSet{
-		opts: opts,
-		dirs: make(map[string]bool),
+		opts:  opts,
+		dirs:  make(map[string]bool),
+		names: make(map[string]string),
 	}
 }
 
@@ -309,7 +311,11 @@ func (s *DirWatchSet) canonical(dir string) string {
 }
 
 func (s *DirWatchSet) Set(dir string, recursive bool) {
+	original := dir
 	dir = s.canonical(dir)
+	if _, exists := s.names[dir]; !exists {
+		s.names[dir] = original
+	}
 	s.dirs[dir] = s.dirs[dir] || recursive
 }
 
@@ -329,7 +335,11 @@ func (s *DirWatchSet) Covered(dir string) bool {
 }
 
 func (s *DirWatchSet) Dirs() map[string]bool {
-	return s.dirs
+	dirs := make(map[string]bool, len(s.dirs))
+	for key, recursive := range s.dirs {
+		dirs[s.names[key]] = recursive
+	}
+	return dirs
 }
 
 func (wm *WatchManager) IsPathUnderWatch(path string, opts tspath.ComparePathsOptions) bool {

tsc/internal/execute/watchmanager/watchmanager_test.go

@@ -83,8 +83,8 @@ func TestDirWatchSetCanonicalDedup(t *testing.T) {
 
 	dirs := insensitive.Dirs()
 	assert.Equal(t, len(dirs), 1, "differently-cased dirs must collapse to one entry")
-	_, canonical := dirs["/repo/node_modules/pkgname"]
-	assert.Assert(t, canonical, "Dirs must be keyed by the canonicalized path")
+	_, original := dirs["/repo/Node_Modules/PkgName"]
+	assert.Assert(t, original, "Dirs must retain the original spelling used for registration")
 
 	sensitive := NewDirWatchSet(caseSensitiveOpts)
 	sensitive.Set("/repo/Node_Modules/PkgName", false)

tsc/internal/fswatch/CHANGES.md

@@ -149,6 +149,34 @@ logical root, physical root, event-ID cutoff, and termination state, so
 late-added watches don't receive older queued events and symlinked watch roots
 continue reporting caller-visible paths.
 
+### macOS path comparison
+
+FSEvents and kqueue use the watched volume's case sensitivity, queried with
+`pathconf`, rather than assuming event paths have the same spelling as the
+subscription. On case-insensitive volumes, CoreFoundation case folding and NFC
+normalization recognize Unicode aliases, including expansions such as sharp s /
+`SS` and ligatures / letter sequences. This is not width- or
+diacritic-insensitive comparison.
+
+Folded forms are comparison keys, never displayed or opened paths. Watch roots
+and subscribed filenames are normalized to NFC. Directory events retain the
+caller's root casing, with NFC suffixes for FSEvents and on-disk child spellings
+for kqueue; `WatchFile` events use the subscribed NFC filename. Rebasing uses
+original path boundaries rather than folded byte lengths. FSEvents routing,
+shared callback filtering, overflow matching, and logical-root deletion use the
+same comparison rules.
+
+An allocation-free ASCII comparison fast path avoids native folding. Watch-root
+comparison forms are prepared at subscription time, while event paths are
+folded lazily and reused across routing comparisons and within callback
+filtering passes. `WatchFile` reuses its parent subscription's comparer rather
+than querying filesystem case sensitivity twice.
+
+The native fold has been compared with aliases and distinct names on
+case-insensitive APFS, but is not a guarantee of identical lookup tables on every
+filesystem or macOS version. Case-sensitive comparison and watcher backends on
+other platforms remain unchanged.
+
 ## New backends
 
 **fanotify** (Linux, kernel ≥ 5.13) is the default on Linux when available. It

tsc/internal/fswatch/README.md

@@ -90,9 +90,20 @@ if errors.Is(err, fswatch.ErrWatchTerminated) {
 - Event order within a batch is **not guaranteed**.
 - The callback runs on a library goroutine, not the caller's. Each watch's
   callback is serialized (never concurrent with itself).
-- Paths in events are absolute. **Resolve symlinks before subscribing**;
-  backends report canonical paths:
+- Paths in events are absolute. Subscribing through a directory symlink follows
+  its target while preserving the caller-visible root in delivered paths.
 
-  ```go
-  realDir, err := filepath.EvalSymlinks(dir)
-  ```
+On macOS, watch roots and subscribed filenames are normalized to NFC. On volumes
+reporting case-insensitive lookup, FSEvents and kqueue match paths using
+CoreFoundation's case-insensitive fold, including expansions such as sharp s /
+`SS` and ligatures / letter sequences. This is not width- or
+diacritic-insensitive comparison. Folded forms are only comparison keys:
+directory events retain the caller's root casing, with an NFC suffix for
+FSEvents and the on-disk child spelling for kqueue; file events use the
+subscribed NFC filename. Symlink-root subscriptions likewise retain the
+caller-visible root.
+
+The fold has been compared with actual aliases and distinct names on
+case-insensitive APFS. It is not a guarantee of identical Unicode lookup
+tables on every filesystem or macOS version. Case-sensitive volumes and
+watcher backends on other platforms retain exact comparison.

tsc/internal/fswatch/canonicalize_darwin.go

@@ -2,13 +2,35 @@
 
 package fswatch
 
-// canonicalizePath returns the path in the form the library uses for
-// internal bookkeeping and event delivery. On macOS, paths from FSEvents
-// arrive using whatever Unicode normalization form is stored on disk;
-// usually NFC, but sometimes NFD (e.g. files created on legacy HFS+
-// volumes or copied from systems that use NFD). APFS resolves either form
-// to the same inode, but raw string comparisons against caller-supplied
-// paths (typically NFC) silently break. Normalizing every path the
-// library ingests to NFC keeps watch keys, dirWatch lookups, WatchFile
-// filters, and event paths all in one consistent form.
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// canonicalizePath normalizes watch keys, subscribed filenames, and incoming
+// FSEvents paths to NFC. kqueue retains on-disk child spellings for its fd
+// bookkeeping and directory events; on case-insensitive volumes, the native
+// path comparer handles normalization differences when filtering WatchFile.
 func canonicalizePath(p string) string { return normalizeNFC(p) }
+
+func (w *watcher) pathComparer(dir string) (pathComparer, error) {
+	if w.name != "fsevents" && w.name != "kqueue" {
+		return pathComparer{}, nil
+	}
+	c, err := PathComparerForPath(dir)
+	return c.comparer, err
+}
+
+// PathComparerForPath queries an existing path's volume. Errors are returned to
+// the caller; a failed query must not silently enable or disable native folding.
+func PathComparerForPath(path string) (PathComparer, error) {
+	// _PC_CASE_SENSITIVE from sys/unistd.h. Query the watched volume rather
+	// than assuming every volume mounted on macOS is case-insensitive.
+	const pcCaseSensitive = 11
+	sensitive, err := unix.Pathconf(path, pcCaseSensitive)
+	if err != nil {
+		return PathComparer{}, &os.PathError{Op: "pathconf", Path: path, Err: err}
+	}
+	return PathComparer{comparer: pathComparer{ignoreCase: sensitive == 0}}, nil
+}

tsc/internal/fswatch/canonicalize_other.go

@@ -2,7 +2,23 @@
 
 package fswatch
 
+const nativePathFolding = false
+
+func foldNativePath(string) string {
+	panic("fswatch: native path folding is only available on Darwin")
+}
+
 // canonicalizePath is a no-op on platforms whose watchers report paths
 // using the same bytes the caller provided. See canonicalize_darwin.go
 // for the rationale on macOS.
 func canonicalizePath(p string) string { return p }
+
+func (w *watcher) pathComparer(dir string) (pathComparer, error) {
+	return pathComparer{}, nil
+}
+
+// PathComparerForPath returns exact comparison on platforms without native
+// Darwin watch aliases. It does not inspect the host filesystem.
+func PathComparerForPath(path string) (PathComparer, error) {
+	return PathComparer{}, nil
+}

tsc/internal/fswatch/fsevents_darwin.go

@@ -507,6 +507,7 @@ func fsEventsCallback(cb *streamCallback, payload *fsEventsCallbackPayload) {
 		if path == "" {
 			continue
 		}
+		comparison := comparisonPath{path: path}
 
 		isRemoved := flag&flagItemRemoved != 0
 		isRenamed := flag&flagItemRenamed != 0
@@ -527,7 +528,7 @@ func fsEventsCallback(cb *streamCallback, payload *fsEventsCallbackPayload) {
 				if watch.state.terminated.Load() {
 					continue
 				}
-				if fseventsOverflowMatches(watch.w, path) {
+				if fseventsOverflowMatchesPrepared(watch.w, &comparison) {
 					watch.w.events.setError(overflow)
 					touched[watch.w] = struct{}{}
 				}
@@ -551,7 +552,7 @@ func fsEventsCallback(cb *streamCallback, payload *fsEventsCallbackPayload) {
 				continue
 			}
 			w := watch.w
-			displayPath, ok := fseventsDisplayPath(w, rawPath)
+			displayPath, ok := fseventsDisplayPathPrepared(w, &comparison)
 			if !ok {
 				continue
 			}
@@ -623,18 +624,42 @@ func fsEventsCallback(cb *streamCallback, payload *fsEventsCallbackPayload) {
 }
 
 func fseventsDisplayPath(w *dirWatch, rawPath string) (string, bool) {
-	if isInDirectoryOrSelf(w.physicalDir, rawPath) {
-		return w.displayPath(rawPath), true
+	path := comparisonPath{path: rawPath}
+	return fseventsDisplayPathPrepared(w, &path)
+}
+
+func fseventsDisplayPathPrepared(w *dirWatch, rawPath *comparisonPath) (string, bool) {
+	physical := comparisonPath{path: w.physicalDir, folded: w.physicalDirFold, ready: w.physicalDirFold != ""}
+	if path, ok := w.comparer.rebasePrepared(rawPath, physical, w.dir); ok {
+		return path, true
 	}
-	if w.physicalDir != w.dir && isInDirectoryOrSelf(w.dir, rawPath) {
-		return rawPath, true
+	if w.physicalDir != w.dir {
+		logical := comparisonPath{path: w.dir, folded: w.dirFold, ready: w.dirFold != ""}
+		return w.comparer.rebasePrepared(rawPath, logical, w.dir)
 	}
 	return "", false
 }
 
 func fseventsOverflowMatches(w *dirWatch, rawPath string) bool {
-	if isInDirectoryOrSelf(w.physicalDir, rawPath) || isInDirectoryOrSelf(rawPath, w.physicalDir) {
+	path := comparisonPath{path: rawPath}
+	return fseventsOverflowMatchesPrepared(w, &path)
+}
+
+func fseventsOverflowMatchesPrepared(w *dirWatch, rawPath *comparisonPath) bool {
+	physical := comparisonPath{path: w.physicalDir, folded: w.physicalDirFold, ready: w.physicalDirFold != ""}
+	if _, ok := w.comparer.suffixPrepared(physical, rawPath); ok {
 		return true
 	}
-	return w.physicalDir != w.dir && (isInDirectoryOrSelf(w.dir, rawPath) || isInDirectoryOrSelf(rawPath, w.dir))
+	if _, ok := w.comparer.suffixPrepared(*rawPath, &physical); ok {
+		return true
+	}
+	if w.physicalDir != w.dir {
+		logical := comparisonPath{path: w.dir, folded: w.dirFold, ready: w.dirFold != ""}
+		if _, ok := w.comparer.suffixPrepared(logical, rawPath); ok {
+			return true
+		}
+		_, ok := w.comparer.suffixPrepared(*rawPath, &logical)
+		return ok
+	}
+	return false
 }

tsc/internal/fswatch/fsevents_darwin_ffi.go

@@ -8,7 +8,9 @@ import (
 	"os"
 	"runtime"
 	"slices"
+	"strings"
 	"syscall"
+	"unicode/utf8"
 	"unsafe"
 
 	"golang.org/x/sys/unix"
@@ -150,13 +152,16 @@ func cfArrayGetValueAtIndex(array uintptr, index int) uintptr {
 // FSEvents reports paths using whatever bytes are stored on disk. APFS is
 // normalization-insensitive for lookups (a file created as NFD opens fine
 // under the NFC form, and vice versa) but it stores and reports the original
-// bytes. The library normalizes every path that crosses the darwin boundary
-// to Unicode NFC so that:
+// bytes. The library normalizes watch paths and incoming FSEvents paths to
+// Unicode NFC so that:
 //   - WatchDirectory("/.../caf\u00e9") and WatchDirectory("/.../cafe\u0301")
 //     coalesce to a single dir watch;
-//   - WatchFile filters by exact-string compare in NFC always match;
+//   - WatchFile filters and directory routing compare the same normalized paths;
 //   - subscribers can compare event paths against their own NFC strings.
 //
+// kqueue retains on-disk child spellings; its WatchFile comparisons also use
+// the native fold below on volumes reporting case-insensitive lookup.
+//
 // All-ASCII inputs are bit-identical in NFC and NFD, so the hot path skips
 // the FFI entirely. The rare non-ASCII case round-trips through CoreFoundation
 // (UTF-8 → CFString → CFMutableString → CFStringNormalize → UTF-8) with no Go
@@ -183,6 +188,46 @@ func cfStringNormalize(mutStr uintptr, form uintptr) {
 	_, _, _ = syscall_syscall6(fse_CFStringNormalize_trampoline_addr, mutStr, form, 0, 0, 0, 0)
 }
 
+//go:cgo_import_dynamic fse_CFStringFold CFStringFold "/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation"
+
+var fse_CFStringFold_trampoline_addr uintptr
+
+const nativePathFolding = true
+
+// foldNativePath is a comparison form, never a displayed or opened path.
+// Case folding expands sharp s and ligatures without making diacritics,
+// dotless i, circled letters, or character widths interchangeable.
+func foldNativePath(s string) string {
+	if isASCII(s) {
+		return strings.ToLower(s)
+	}
+	if !utf8.ValidString(s) || strings.IndexByte(s, 0) >= 0 {
+		return ""
+	}
+	cstr := append([]byte(s), 0)
+	src := cfStringCreate(0, unsafe.Pointer(&cstr[0]), cfStringEncodingUTF8)
+	if src == 0 {
+		panic("fswatch: cannot create CFString for path folding")
+	}
+	defer cfRelease(src)
+	mut := cfStringCreateMutableCopy(0, 0, src)
+	if mut == 0 {
+		panic("fswatch: cannot copy CFString for path folding")
+	}
+	defer cfRelease(mut)
+	// Normalize before folding as well: a decomposed capital I with dot
+	// must have the same comparison form as precomposed dotted capital I.
+	cfStringNormalize(mut, cfStringNormalizationFormC)
+	const cfCompareCaseInsensitive = 1
+	_, _, _ = syscall_syscall6(fse_CFStringFold_trampoline_addr, mut, cfCompareCaseInsensitive, 0, 0, 0, 0)
+	cfStringNormalize(mut, cfStringNormalizationFormC)
+	folded := cfStringToGo(mut)
+	if folded == "" {
+		panic("fswatch: cannot extract folded CFString")
+	}
+	return folded
+}
+
 //go:cgo_import_dynamic fse_CFStringGetLength CFStringGetLength "/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation"
 
 var fse_CFStringGetLength_trampoline_addr uintptr

tsc/internal/fswatch/fsevents_darwin_ffi.s

@@ -62,6 +62,12 @@ TEXT fse_CFStringNormalize_trampoline<>(SB), NOSPLIT, $0-0
 GLOBL ·fse_CFStringNormalize_trampoline_addr(SB), RODATA, $8
 DATA ·fse_CFStringNormalize_trampoline_addr(SB)/8, $fse_CFStringNormalize_trampoline<>(SB)
 
+TEXT fse_CFStringFold_trampoline<>(SB), NOSPLIT, $0-0
+	JMP fse_CFStringFold(SB)
+
+GLOBL ·fse_CFStringFold_trampoline_addr(SB), RODATA, $8
+DATA ·fse_CFStringFold_trampoline_addr(SB)/8, $fse_CFStringFold_trampoline<>(SB)
+
 TEXT fse_CFStringGetLength_trampoline<>(SB), NOSPLIT, $0-0
 	JMP fse_CFStringGetLength(SB)
 

tsc/internal/fswatch/fsevents_darwin_nfd_test.go

@@ -3,9 +3,12 @@
 package fswatch
 
 import (
+	"errors"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
+	"time"
 )
 
 // These tests document a real cross-normalization failure mode on macOS:
@@ -174,3 +177,439 @@ func TestFSEventsNFDOnDiskNFCWatchFile(t *testing.T) {
 		}
 	}
 }
+
+// Check identity independently of the comparer, including exclusive creation.
+// Filesystems that do not support a particular alias cannot exercise its watch.
+func requireDarwinAlias(t *testing.T, a, b string) {
+	t.Helper()
+	first, err := os.Stat(a)
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := os.Stat(b)
+	if errors.Is(err, os.ErrNotExist) {
+		t.Skip("filesystem does not alias these spellings")
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !os.SameFile(first, second) {
+		t.Fatal("alternate spelling resolved to a different inode")
+	}
+	if err := os.Mkdir(b, 0o755); !errors.Is(err, os.ErrExist) {
+		t.Fatalf("exclusive alternate creation: %v", err)
+	}
+}
+
+func TestDarwinWatchFileComparison(t *testing.T) {
+	t.Parallel()
+	cases := []struct {
+		name                string
+		diskRoot, watchRoot string
+		diskFile, watchFile string
+		alias               bool
+	}{
+		{"root-case", "Mixed", "mixed", "file.ts", "file.ts", true},
+		{"leaf-case", "root", "root", "File.ts", "file.ts", true},
+		{"sharp-s", "root", "root", "\u00df.ts", "SS.ts", true},
+		{"dotted-i", "root", "root", "\u0130.ts", "i\u0307.ts", true},
+		{"ligature", "root", "root", "\ufb03.ts", "ffi.ts", true},
+		{"normalization", "root", "root", "cafe\u0301.ts", "caf\u00e9.ts", true},
+		{"dotless-i", "root", "root", "I.ts", "\u0131.ts", false},
+		{"ascii-dotted-i", "root", "root", "i.ts", "\u0130.ts", false},
+		{"fullwidth", "root", "root", "\uff21.ts", "A.ts", false},
+		{"sibling", "root", "root", "file.ts2", "file.ts", false},
+		{"case-sensitive", "root", "root", "File.ts", "file.ts", false},
+	}
+	for _, impl := range []Watcher{Kqueue(), FSEvents()} {
+		for _, c := range cases {
+			t.Run(impl.Name()+"/"+c.name, func(t *testing.T) {
+				t.Parallel()
+				for _, reverse := range []bool{false, true} {
+					diskRoot, watchRoot := c.diskRoot, c.watchRoot
+					diskName, watchName := c.diskFile, c.watchFile
+					if reverse {
+						diskRoot, watchRoot = watchRoot, diskRoot
+						diskName, watchName = watchName, diskName
+					}
+					parent := newTmpDir(t)
+					diskRoot, watchRoot = filepath.Join(parent, diskRoot), filepath.Join(parent, watchRoot)
+					if err := os.Mkdir(diskRoot, 0o755); err != nil {
+						t.Fatal(err)
+					}
+					requireDarwinAlias(t, diskRoot, watchRoot)
+					diskFile, watchFile := filepath.Join(diskRoot, diskName), filepath.Join(watchRoot, watchName)
+					if err := os.WriteFile(diskFile, nil, 0o644); err != nil {
+						t.Fatal(err)
+					}
+					if c.alias {
+						requireDarwinAlias(t, diskFile, watchFile)
+					} else {
+						f, err := os.OpenFile(watchFile, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
+						if errors.Is(err, os.ErrExist) {
+							t.Skip("filesystem aliases these spellings")
+						}
+						if err != nil {
+							t.Fatal(err)
+						}
+						if err = f.Close(); err != nil {
+							t.Fatal(err)
+						}
+						a, err := os.Stat(diskFile)
+						if err != nil {
+							t.Fatal(err)
+						}
+						b, err := os.Stat(watchFile)
+						if err != nil || os.SameFile(a, b) {
+							t.Fatalf("expected distinct inodes: %v", err)
+						}
+					}
+					// Subscribe while the target is absent to exercise discovery
+					// as well as subsequent fd-based updates and deletion.
+					if err := os.Remove(diskFile); err != nil {
+						t.Fatal(err)
+					}
+					dir, _ := subscribeForOpts(t, watchRoot, impl)
+					file, _ := subscribeFileFor(t, watchFile, impl)
+					wantDir := filepath.Join(watchRoot, diskName)
+					if impl == FSEvents() {
+						wantDir = canonicalizePath(wantDir)
+					}
+					for round := range 3 {
+						kind := EventUpdate
+						if round == 2 {
+							kind = EventDelete
+							if err := os.Remove(diskFile); err != nil {
+								t.Fatal(err)
+							}
+						} else if err := os.WriteFile(diskFile, make([]byte, round+1), 0o644); err != nil {
+							t.Fatal(err)
+						}
+						expectContains(t, dir, kind, wantDir)
+						if c.alias {
+							events := expectContains(t, file, kind, canonicalizePath(watchFile))
+							for _, e := range events {
+								if e.Path != canonicalizePath(watchFile) {
+									t.Fatalf("unexpected file event spelling: %q", e.Path)
+								}
+							}
+						} else if events := file.next(400 * time.Millisecond); len(events) != 0 {
+							t.Fatalf("cross-routed distinct filename: %v", events)
+						}
+					}
+				}
+			})
+		}
+	}
+}
+
+func TestNativePathComparerKeys(t *testing.T) {
+	t.Parallel()
+	c := PathComparer{comparer: pathComparer{ignoreCase: true}}
+	for _, pair := range [][2]string{
+		{"/A/file.ts", "/a/FILE.TS"},
+		{"/straße/İ.ts", "/STRASSE/i\u0307.ts"},
+		{"/\ufb03/ſ.ts", "/ffi/S.ts"},
+		{"/Σ/ς.ts", "/σ/σ.ts"},
+		{"/cafe\u0301.ts", "/caf\u00e9.ts"},
+	} {
+		if c.Key(pair[0]) != c.Key(pair[1]) {
+			t.Errorf("unequal keys for aliases %q", pair)
+		}
+	}
+	for _, pair := range [][2]string{
+		{"/I.ts", "/\u0131.ts"},
+		{"/i.ts", "/İ.ts"},
+		{"/Ａ.ts", "/A.ts"},
+		{"/file.ts2", "/file.ts"},
+	} {
+		if c.Key(pair[0]) == c.Key(pair[1]) {
+			t.Errorf("equal keys for distinct paths %q", pair)
+		}
+	}
+	for _, name := range []string{"/A\xff.ts", "/A\x00.ts"} {
+		if got := c.Key(name); got != name {
+			t.Errorf("malformed path was folded: %q => %q", name, got)
+		}
+	}
+	if got, ok := c.Rebase("/STRASSE/new.ts", "/straße", "/original"); !ok || got != "/original/new.ts" {
+		t.Fatalf("expanding fold rebased at wrong boundary: %q, %v", got, ok)
+	}
+}
+
+func TestPathComparerVolumeQueryError(t *testing.T) {
+	t.Parallel()
+	name := "./watcher_test.go/missing"
+	_, err := PathComparerForPath(name)
+	var pathErr *os.PathError
+	if !errors.As(err, &pathErr) || pathErr.Op != "pathconf" || pathErr.Path != name {
+		t.Fatalf("volume query did not preserve path error: %v", err)
+	}
+}
+
+func TestFSEventsDifferentCasing(t *testing.T) {
+	t.Parallel()
+
+	for _, recursive := range []bool{false, true} {
+		name := "nonrecursive"
+		if recursive {
+			name = "recursive"
+		}
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			parent := newTmpDir(t)
+			diskDir := filepath.Join(parent, "MixedCase")
+			watchDir := filepath.Join(parent, "mixedcase")
+			if err := os.Mkdir(diskDir, 0o755); err != nil {
+				t.Fatal(err)
+			}
+			if _, err := os.Stat(watchDir); errors.Is(err, os.ErrNotExist) {
+				t.Skip("requires a case-insensitive filesystem")
+			} else if err != nil {
+				t.Fatal(err)
+			}
+
+			var opts []WatchOption
+			if recursive {
+				opts = append(opts, WithRecursive())
+			}
+			r, _ := subscribeForOpts(t, watchDir, FSEvents(), opts...)
+			file := filepath.Join(diskDir, "File.ts")
+			want := filepath.Join(watchDir, "File.ts")
+			if err := os.WriteFile(file, []byte("export {}"), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			expectContains(t, r, EventUpdate, want)
+			if err := os.Remove(file); err != nil {
+				t.Fatal(err)
+			}
+			expectContains(t, r, EventDelete, want)
+		})
+	}
+}
+
+func TestFSEventsWatchFileDifferentCasing(t *testing.T) {
+	t.Parallel()
+	dir := newTmpDir(t)
+	diskFile := filepath.Join(dir, "File.ts")
+	watchFile := filepath.Join(dir, "file.ts")
+	if err := os.WriteFile(diskFile, []byte("export {}"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(watchFile); errors.Is(err, os.ErrNotExist) {
+		t.Skip("requires a case-insensitive filesystem")
+	} else if err != nil {
+		t.Fatal(err)
+	}
+	r, _ := subscribeFileFor(t, watchFile, FSEvents())
+	if err := os.WriteFile(diskFile, []byte("export const x = 1;"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	expectContains(t, r, EventUpdate, watchFile)
+	if err := os.Remove(diskFile); err != nil {
+		t.Fatal(err)
+	}
+	expectContains(t, r, EventDelete, watchFile)
+
+	missingFile := filepath.Join(dir, "missing.ts")
+	missing, _ := subscribeFileFor(t, missingFile, FSEvents())
+	if err := os.WriteFile(filepath.Join(dir, "Missing.ts"), []byte("export {}"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	expectContains(t, missing, EventUpdate, missingFile)
+}
+
+var fseventsFoldPairs = []struct {
+	name  string
+	a, b  string
+	alias bool
+}{
+	{"sharp-s", "\u00df", "SS", true},
+	{"capital-sharp-s", "\u1e9e", "SS", true},
+	{"dotted-i", "\u0130", "i\u0307", true},
+	{"ligature-ff", "\ufb00", "ff", true},
+	{"ligature-ffi", "\ufb03", "ffi", true},
+	{"long-s", "\u017f", "S", true},
+	{"sigma", "\u03c2", "\u03a3", true},
+	{"accent", "\u00e9", "E\u0301", true},
+	{"dotless-i", "I", "\u0131", false},
+	{"ascii-dotted-i", "i", "\u0130", false},
+	{"ascii-i-dot", "I", "i\u0307", false},
+	{"circled-a", "\u24d0", "a", false},
+	{"fullwidth-a", "\uff21", "A", false},
+}
+
+func TestNativePathFold(t *testing.T) {
+	t.Parallel()
+	for _, pair := range fseventsFoldPairs {
+		a, b := foldNativePath(pair.a), foldNativePath(pair.b)
+		if (a == b) != pair.alias {
+			t.Errorf("%s: folds %q / %q, alias=%v", pair.name, a, b, pair.alias)
+		}
+		if a != foldNativePath(canonicalizePath(pair.a)) || b != foldNativePath(canonicalizePath(pair.b)) {
+			t.Errorf("%s: normalization changed folding", pair.name)
+		}
+	}
+	for _, input := range []string{"\u0130", "I\u0307", "i\u0307"} {
+		if got := foldNativePath(input); got != "i\u0307" {
+			t.Errorf("fold(%q) = %q", input, got)
+		}
+	}
+	for _, input := range []string{"/\xff", "/\xfe", "/\u00df\x00suffix"} {
+		if got := foldNativePath(input); got != "" {
+			t.Errorf("invalid native path %q produced %q", input, got)
+		}
+	}
+	c := pathComparer{ignoreCase: true}
+	for _, pair := range [][2]string{{"/SS/", "/\u00df/File.ts"}, {"/\u00df/", "/SS/File.ts"}} {
+		if suffix, ok := c.suffix(pair[0], pair[1]); !ok || suffix != "File.ts" {
+			t.Errorf("trailing separator: got (%q, %v)", suffix, ok)
+		}
+	}
+}
+
+func TestFSEventsExpansionAliases(t *testing.T) {
+	t.Parallel()
+	for _, pair := range fseventsFoldPairs {
+		if !pair.alias {
+			continue
+		}
+		t.Run(pair.name, func(t *testing.T) {
+			t.Parallel()
+			for _, reverse := range []bool{false, true} {
+				a, b := pair.a, pair.b
+				if reverse {
+					a, b = b, a
+				}
+				parent := newTmpDir(t)
+				disk, root := filepath.Join(parent, a), filepath.Join(parent, b)
+				if err := os.Mkdir(disk, 0o755); err != nil {
+					t.Fatal(err)
+				}
+				requireDarwinAlias(t, disk, root)
+				nested := filepath.Join(disk, "Nested")
+				if err := os.Mkdir(nested, 0o755); err != nil {
+					t.Fatal(err)
+				}
+				direct, _ := subscribeForOpts(t, root, FSEvents())
+				recursive, _ := subscribeFor(t, root, FSEvents())
+				link := filepath.Join(parent, "Link")
+				makeDirSymlink(t, root, link)
+				linked, _ := subscribeFor(t, link, FSEvents())
+				file, _ := subscribeFileFor(t, filepath.Join(root, b+".ts"), FSEvents())
+				control, _ := subscribeFileFor(t, filepath.Join(disk, a+".ts"), FSEvents())
+				diskFile := filepath.Join(disk, a+".ts")
+				for round := range 3 {
+					kind := EventUpdate
+					if round == 2 {
+						kind = EventDelete
+						if err := os.Remove(diskFile); err != nil {
+							t.Fatal(err)
+						}
+					} else if err := os.WriteFile(diskFile, []byte(strings.Repeat("x", round+1)), 0o644); err != nil {
+						t.Fatal(err)
+					}
+					expectContains(t, control, kind, canonicalizePath(diskFile))
+					expectContains(t, file, kind, canonicalizePath(filepath.Join(root, b+".ts")))
+					expectContains(t, direct, kind, canonicalizePath(filepath.Join(root, a+".ts")))
+					expectContains(t, recursive, kind, canonicalizePath(filepath.Join(root, a+".ts")))
+					expectContains(t, linked, kind, canonicalizePath(filepath.Join(link, a+".ts")))
+				}
+				child := filepath.Join(nested, "File.ts")
+				if err := os.WriteFile(child, nil, 0o644); err != nil {
+					t.Fatal(err)
+				}
+				expectContains(t, recursive, EventUpdate, canonicalizePath(filepath.Join(root, "Nested", "File.ts")))
+				expectContains(t, linked, EventUpdate, filepath.Join(link, "Nested", "File.ts"))
+				if events := direct.next(400 * time.Millisecond); len(events) != 0 {
+					t.Fatalf("nonrecursive watch received nested events: %v", events)
+				}
+				if err := os.Remove(child); err != nil {
+					t.Fatal(err)
+				}
+				expectContains(t, recursive, EventDelete, canonicalizePath(filepath.Join(root, "Nested", "File.ts")))
+				if err := os.Remove(nested); err != nil {
+					t.Fatal(err)
+				}
+				expectContains(t, direct, EventDelete, canonicalizePath(filepath.Join(root, "Nested")))
+				if err := os.Remove(disk); err != nil {
+					t.Fatal(err)
+				}
+				expectContains(t, recursive, EventDelete, canonicalizePath(root))
+				terminated := false
+				deadline := time.Now().Add(direct.deadline())
+				for !terminated && time.Now().Before(deadline) {
+					direct.mu.Lock()
+					for _, err := range direct.errs {
+						terminated = terminated || errors.Is(err, ErrWatchTerminated)
+					}
+					direct.mu.Unlock()
+					if !terminated {
+						time.Sleep(20 * time.Millisecond)
+					}
+				}
+				if !terminated {
+					t.Fatal("missing root termination")
+				}
+			}
+		})
+	}
+}
+
+func TestFSEventsFoldDistinctNames(t *testing.T) {
+	t.Parallel()
+	for _, pair := range fseventsFoldPairs {
+		if pair.alias {
+			continue
+		}
+		t.Run(pair.name, func(t *testing.T) {
+			t.Parallel()
+			for _, reverse := range []bool{false, true} {
+				parent := newTmpDir(t)
+				roots := []string{filepath.Join(parent, pair.a), filepath.Join(parent, pair.b)}
+				if reverse {
+					roots[0], roots[1] = roots[1], roots[0]
+				}
+				recorders := make([]*recordingWatcher, 2)
+				files := make([]*recordingWatcher, 2)
+				for i, root := range roots {
+					if err := os.Mkdir(root, 0o755); err != nil {
+						t.Fatal(err)
+					}
+					recorders[i], _ = subscribeFor(t, root, FSEvents())
+					files[i], _ = subscribeFileFor(t, root+".ts", FSEvents())
+				}
+				a, err := os.Stat(roots[0])
+				if err != nil {
+					t.Fatal(err)
+				}
+				b, err := os.Stat(roots[1])
+				if err != nil || os.SameFile(a, b) {
+					t.Fatalf("expected distinct inodes: %v", err)
+				}
+				child := filepath.Join(roots[0], "File.ts")
+				file := roots[0] + ".ts"
+				for round := range 3 {
+					kind := EventUpdate
+					for _, path := range []string{child, file} {
+						if round == 2 {
+							kind = EventDelete
+							if err := os.Remove(path); err != nil {
+								t.Fatal(err)
+							}
+						} else if err := os.WriteFile(path, []byte(strings.Repeat("x", round+1)), 0o644); err != nil {
+							t.Fatal(err)
+						}
+					}
+					expectContains(t, recorders[0], kind, canonicalizePath(child))
+					expectContains(t, files[0], kind, canonicalizePath(file))
+					for _, r := range []*recordingWatcher{recorders[1], files[1]} {
+						if events := r.next(400 * time.Millisecond); len(events) != 0 {
+							t.Fatalf("cross-routed distinct name: %v", events)
+						}
+					}
+				}
+			}
+		})
+	}
+}

tsc/internal/fswatch/fsevents_darwin_shared_test.go

@@ -8,6 +8,8 @@ import (
 	"os"
 	"path/filepath"
 	"slices"
+	"strconv"
+	"strings"
 	"testing"
 	"time"
 )
@@ -290,3 +292,314 @@ func TestFSEventsOverflowMatchesWatch(t *testing.T) {
 		})
 	}
 }
+
+func TestFSEventsCaseSensitiveRouting(t *testing.T) {
+	t.Parallel()
+	for _, ignoreCase := range []bool{false, true} {
+		w := &dirWatch{
+			dir:         "/logical/root",
+			physicalDir: "/physical/root",
+			comparer:    pathComparer{ignoreCase: ignoreCase},
+		}
+		for _, root := range []string{"/PHYSICAL/ROOT", "/LOGICAL/ROOT"} {
+			for _, suffix := range []string{"", "/File.ts", "/Nested/File.ts"} {
+				path, ok := fseventsDisplayPath(w, root+suffix)
+				if ok != ignoreCase || ok && path != w.dir+suffix {
+					t.Errorf("display path for %q, ignoreCase=%v: got (%q, %v)", root+suffix, ignoreCase, path, ok)
+				}
+			}
+			if fseventsOverflowMatches(w, root+"/Nested") != ignoreCase {
+				t.Errorf("overflow descendant %q, ignoreCase=%v", root, ignoreCase)
+			}
+			if fseventsOverflowMatches(w, filepath.Dir(root)) != ignoreCase {
+				t.Errorf("overflow ancestor %q, ignoreCase=%v", root, ignoreCase)
+			}
+			if _, ok := fseventsDisplayPath(w, root+"2/File.ts"); ok {
+				t.Errorf("matched sibling of %q, ignoreCase=%v", root, ignoreCase)
+			}
+			if fseventsOverflowMatches(w, root+"2") {
+				t.Errorf("overflow matched sibling of %q, ignoreCase=%v", root, ignoreCase)
+			}
+		}
+	}
+}
+
+func TestFSEventsConsolidatedDifferentCasing(t *testing.T) {
+	t.Parallel()
+	for _, recursive := range []bool{false, true} {
+		dw := newDirectWatcher(t, "/parent")
+		dw.comparer = pathComparer{ignoreCase: true}
+		child := "/parent/child"
+		var got []Event
+		var gotErr error
+		dw.watch(child, child, recursive, func(events []Event, err error) {
+			got = append(got, events...)
+			gotErr = err
+		}, func(path string) bool {
+			return path == child+"/Ignored.ts"
+		})
+		dw.events.update("/parent/CHILD/File.ts")
+		dw.events.update("/parent/CHILD/Ignored.ts")
+		dw.events.update("/parent/CHILD2/File.ts")
+		dw.events.update("/parent/CHILD/Nested/File.ts")
+		dw.triggerCallbacks()
+		wantCount := 1
+		if recursive {
+			wantCount++
+		}
+		if len(got) != wantCount || gotErr != nil {
+			t.Fatalf("recursive=%v: got events=%v, err=%v", recursive, got, gotErr)
+		}
+		for _, e := range got {
+			if e.Path != child+"/File.ts" && e.Path != child+"/Nested/File.ts" {
+				t.Fatalf("unexpected path %q", e.Path)
+			}
+		}
+		got = nil
+		if !dw.terminateCallbacksForDeletedRoot("/parent/CHILD", 1, ErrWatchTerminated) {
+			t.Fatal("expected differently cased child root to terminate")
+		}
+		dw.events.removeWatchRootAt("/parent/CHILD", 1)
+		dw.triggerCallbacks()
+		if !errors.Is(gotErr, ErrWatchTerminated) || len(got) != 1 || got[0].Kind != EventDelete || got[0].Path != child {
+			t.Fatalf("expected child deletion and termination: got events=%v, err=%v", got, gotErr)
+		}
+	}
+}
+
+func TestFSEventsConsolidatedExpansion(t *testing.T) {
+	t.Parallel()
+	for _, pair := range fseventsFoldPairs {
+		for _, reverse := range []bool{false, true} {
+			a, b := pair.a, pair.b
+			if reverse {
+				a, b = b, a
+			}
+			for _, recursive := range []bool{false, true} {
+				dw := newDirectWatcher(t, "/parent")
+				dw.setComparer(pathComparer{ignoreCase: true})
+				child, raw := "/parent/"+canonicalizePath(a), "/parent/"+canonicalizePath(b)
+				var got []Event
+				var gotErr error
+				dw.watch(child, child, recursive, func(events []Event, err error) {
+					got = append(got, events...)
+					gotErr = err
+				}, func(path string) bool { return path == child+"/Ignored.ts" })
+				dw.events.update(raw + "/File.ts")
+				dw.events.update(raw + "/Nested/File.ts")
+				dw.events.update(raw + "/Ignored.ts")
+				dw.events.update(raw + "2/File.ts")
+				dw.triggerCallbacks()
+				want := 0
+				if pair.alias {
+					want = 1
+					if recursive {
+						want++
+					}
+				}
+				if len(got) != want || gotErr != nil {
+					t.Fatalf("%s reverse=%v recursive=%v: got %v, %v", pair.name, reverse, recursive, got, gotErr)
+				}
+				for _, e := range got {
+					if e.Path != child+"/File.ts" && e.Path != child+"/Nested/File.ts" {
+						t.Fatalf("incorrect rebasing: %v", e)
+					}
+				}
+				got = nil
+				if dw.terminateCallbacksForDeletedRoot(raw, 1, ErrWatchTerminated) != pair.alias {
+					t.Fatalf("%s: incorrect termination", pair.name)
+				}
+				dw.events.removeWatchRootAt(raw, 1)
+				dw.triggerCallbacks()
+				if pair.alias && (len(got) != 1 || got[0].Path != child || got[0].Kind != EventDelete || !errors.Is(gotErr, ErrWatchTerminated)) {
+					t.Fatalf("%s: missing deletion/termination: %v, %v", pair.name, got, gotErr)
+				}
+				if !pair.alias && (len(got) != 0 || gotErr != nil) {
+					t.Fatalf("%s: cross-routed deletion/termination: %v, %v", pair.name, got, gotErr)
+				}
+			}
+		}
+	}
+}
+
+func TestFSEventsExpansionRouting(t *testing.T) {
+	t.Parallel()
+	for _, pair := range fseventsFoldPairs {
+		for _, reverse := range []bool{false, true} {
+			a, b := pair.a, pair.b
+			if reverse {
+				a, b = b, a
+			}
+			for padding := range 16 {
+				root := "/physical/" + strings.Repeat("x", padding) + canonicalizePath(a)
+				raw := "/PHYSICAL/" + strings.Repeat("x", padding) + canonicalizePath(b)
+				for _, ignoreCase := range []bool{false, true} {
+					w := &dirWatch{dir: "/logical/Caller", physicalDir: root, comparer: pathComparer{ignoreCase: ignoreCase}}
+					w.setComparer(w.comparer)
+					want := ignoreCase && pair.alias
+					for _, suffix := range []string{"", "/File\u00df.ts", "/Nested/File.ts"} {
+						got, ok := fseventsDisplayPath(w, raw+suffix)
+						if ok != want || ok && got != w.dir+suffix {
+							t.Fatalf("%s reverse=%v padding=%d ignoreCase=%v: got (%q, %v)", pair.name, reverse, padding, ignoreCase, got, ok)
+						}
+					}
+					if fseventsOverflowMatches(w, raw+"/Nested") != want || fseventsOverflowMatches(w, raw) != want {
+						t.Fatalf("%s: incorrect overflow routing", pair.name)
+					}
+					if _, ok := fseventsDisplayPath(w, raw+"2/File.ts"); ok {
+						t.Fatalf("%s: matched sibling", pair.name)
+					}
+					w.physicalDir += "/Nested"
+					w.setComparer(w.comparer)
+					if fseventsOverflowMatches(w, raw) != want {
+						t.Fatalf("%s: incorrect ancestor overflow routing", pair.name)
+					}
+				}
+			}
+		}
+	}
+}
+
+func TestFSEventsLazyPathFolding(t *testing.T) {
+	t.Parallel()
+	for _, tt := range []struct {
+		root, event string
+		match, fold bool
+	}{
+		{"/root", "/root/File\u00df.ts", true, false},
+		{"/root", "/other/\u00df/File.ts", false, false},
+		{"/ROOT", "/root/File.ts", true, false},
+		{"/SS", "/\u00df/File.ts", true, true},
+		{"/\u00df", "/SS/File.ts", true, true},
+	} {
+		w := &dirWatch{dir: tt.root, physicalDir: tt.root}
+		w.setComparer(pathComparer{ignoreCase: true})
+		event := comparisonPath{path: tt.event}
+		for range 10 {
+			if _, ok := fseventsDisplayPathPrepared(w, &event); ok != tt.match || event.ready != tt.fold {
+				t.Fatalf("root=%q event=%q: match=%v, folded=%v", tt.root, tt.event, ok, event.ready)
+			}
+		}
+	}
+	var cache comparisonCache
+	for _, root := range []string{"/SS", "/ss"} {
+		cb := callback{
+			dir: root, physicalDir: root, comparer: pathComparer{ignoreCase: true},
+			physicalComparison: pathComparer{ignoreCase: true}.prepare(root),
+		}
+		e := cb.mapEventCached(Event{Path: "/\u00df/File.ts", Kind: EventUpdate}, &cache)
+		if e.Path != root+"/File.ts" {
+			t.Fatalf("cached callback: %v", e)
+		}
+	}
+	if len(cache) != 1 || cache["/\u00df/File.ts"] != "/ss/file.ts" {
+		t.Fatalf("expected one shared event comparison, got %v", cache)
+	}
+}
+
+func BenchmarkFSEventsDisplayPath(b *testing.B) {
+	const root = "/Users/developer/work/TypeScript/packages/vscode-typescript"
+	for _, scenario := range []struct {
+		name string
+		root string
+		path string
+		want string
+		ok   bool
+	}{
+		{"exact-match", root, root + "/src/File.ts", root + "/src/File.ts", true},
+		{"case-mismatch", strings.ToLower(root), root + "/src/File.ts", strings.ToLower(root) + "/src/File.ts", true},
+		{"sibling-miss", root, "/Users/developer/work/TypeScript/packages/other-package/src/File.ts", "", false},
+		{"unrelated-miss", root, "/private/tmp/other/File.ts", "", false},
+		{"unicode-match", "/Users/developer/work/caf\u00e9", "/Users/developer/work/CAF\u00c9/File.ts", "/Users/developer/work/caf\u00e9/File.ts", true},
+		{"unicode-length-match", "/Users/developer/work/s", "/Users/developer/work/\u017f/File.ts", "/Users/developer/work/s/File.ts", true},
+		{"expanding-event", "/Users/developer/work/SS", "/Users/developer/work/\u00df/File.ts", "/Users/developer/work/SS/File.ts", true},
+		{"expanding-root", "/Users/developer/work/\u00df", "/Users/developer/work/SS/File.ts", "/Users/developer/work/\u00df/File.ts", true},
+		{"unicode-unrelated-miss", root, "/private/tmp/\u00df/File.ts", "", false},
+	} {
+		b.Run(scenario.name, func(b *testing.B) {
+			w := &dirWatch{dir: scenario.root, physicalDir: scenario.root, comparer: pathComparer{ignoreCase: true}}
+			w.setComparer(w.comparer)
+			if got, ok := fseventsDisplayPath(w, scenario.path); got != scenario.want || ok != scenario.ok {
+				b.Fatalf("got (%q, %v), want (%q, %v)", got, ok, scenario.want, scenario.ok)
+			}
+			b.ReportAllocs()
+			for b.Loop() {
+				fseventsDisplayPath(w, scenario.path)
+			}
+		})
+	}
+}
+
+func BenchmarkFSEventsRoutingFanout(b *testing.B) {
+	for _, count := range []int{100, 1000} {
+		watches := make([]dirWatch, count)
+		for i := range watches {
+			dir := fmt.Sprintf("/Users/developer/work/TypeScript/packages/package%04d", i)
+			watches[i] = dirWatch{dir: dir, physicalDir: dir, comparer: pathComparer{ignoreCase: true}}
+			watches[i].setComparer(watches[i].comparer)
+		}
+		path := watches[count-1].dir + "/src/File.ts"
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			matches := 0
+			for i := range watches {
+				if got, ok := fseventsDisplayPath(&watches[i], path); ok {
+					matches++
+					if got != path {
+						b.Fatalf("got %q, want %q", got, path)
+					}
+				}
+			}
+			if matches != 1 {
+				b.Fatalf("got %d matches, want 1", matches)
+			}
+			b.ReportAllocs()
+			for b.Loop() {
+				event := comparisonPath{path: path}
+				for i := range watches {
+					fseventsDisplayPathPrepared(&watches[i], &event)
+				}
+
+			}
+		})
+	}
+}
+
+func BenchmarkFSEventsUnicodeFanout(b *testing.B) {
+	for _, scenario := range []struct{ name, root, event string }{
+		{"simple", "S", "\u017f"},
+		{"expanding-event", "SS", "\u00df"},
+		{"expanding-root", "\u00df", "SS"},
+	} {
+		for _, count := range []int{100, 1000} {
+			b.Run(scenario.name+"/"+strconv.Itoa(count), func(b *testing.B) {
+				watches := make([]dirWatch, count)
+				for i := range watches {
+					dir := fmt.Sprintf("/Users/developer/work/%s/package%04d", scenario.root, i)
+					watches[i] = dirWatch{dir: dir, physicalDir: dir}
+					watches[i].setComparer(pathComparer{ignoreCase: true})
+				}
+				path := fmt.Sprintf("/Users/developer/work/%s/package%04d/File.ts", scenario.event, count-1)
+				matches := 0
+				event := comparisonPath{path: path}
+				for i := range watches {
+					if got, ok := fseventsDisplayPathPrepared(&watches[i], &event); ok {
+						matches++
+						if got != watches[i].dir+"/File.ts" {
+							b.Fatalf("unexpected display path %q", got)
+						}
+					}
+				}
+				if matches != 1 || !event.ready {
+					b.Fatalf("matches=%d, event folded=%v", matches, event.ready)
+				}
+				b.ReportAllocs()
+				for b.Loop() {
+					event := comparisonPath{path: path}
+					for i := range watches {
+						fseventsDisplayPathPrepared(&watches[i], &event)
+					}
+				}
+			})
+		}
+	}
+}

tsc/internal/fswatch/pathcompare.go

@@ -0,0 +1,186 @@
+package fswatch
+
+import (
+	"strings"
+	"unicode/utf8"
+)
+
+type pathComparer struct {
+	ignoreCase bool
+}
+
+// Watch roots are prepared before publication and are immutable thereafter.
+// Event paths are local to one routing operation and folded only on demand.
+type comparisonPath struct {
+	path   string
+	folded string
+	ready  bool
+	cache  *comparisonCache
+}
+
+// Shared only within a synchronous callback/termination pass, never published
+// to a subscriber or stored on a watch.
+type comparisonCache map[string]string
+
+func (c pathComparer) prepare(path string) comparisonPath {
+	p := comparisonPath{path: path}
+	if c.ignoreCase && nativePathFolding {
+		p.fold()
+	}
+	return p
+}
+
+func (p *comparisonPath) fold() string {
+	if !p.ready {
+		if p.cache != nil {
+			if folded, ok := (*p.cache)[p.path]; ok {
+				p.folded, p.ready = folded, true
+				return folded
+			}
+		}
+		p.folded = foldNativePath(p.path)
+		p.ready = true
+		if p.cache != nil {
+			if *p.cache == nil {
+				*p.cache = make(comparisonCache)
+			}
+			(*p.cache)[p.path] = p.folded
+		}
+	}
+	return p.folded
+}
+
+// suffix returns the part of path below root, respecting directory boundaries.
+func (c pathComparer) suffix(root, path string) (string, bool) {
+	p := comparisonPath{path: path}
+	return c.suffixPrepared(comparisonPath{path: root}, &p)
+}
+
+func (c pathComparer) suffixPrepared(root comparisonPath, path *comparisonPath) (string, bool) {
+	if isInDirectoryOrSelf(root.path, path.path) {
+		return path.path[len(root.path):], true
+	}
+	if !c.ignoreCase || root.path == "" {
+		return "", false
+	}
+	suffix, ok, unicode := pathSuffixASCII(root.path, path.path)
+	if !unicode {
+		return suffix, ok
+	}
+	return c.suffixUnicode(root, path)
+}
+
+func (c pathComparer) suffixUnicode(root comparisonPath, path *comparisonPath) (string, bool) {
+	if !nativePathFolding {
+		return pathSuffixFoldUnicode(root.path, path.path)
+	}
+	a, b := root.fold(), path.fold()
+	if a == "" || b == "" {
+		// CFString cannot represent invalid UTF-8. Retain the simple-fold
+		// behavior for malformed paths rather than truncating or losing bytes.
+		return pathSuffixFoldUnicode(root.path, path.path)
+	}
+	if !isInDirectoryOrSelf(a, b) {
+		return "", false
+	}
+	if a == b {
+		return "", true
+	}
+	// Folding and canonical normalization preserve separators, but not byte
+	// lengths. Find the matching boundary in the original event, not its fold.
+	offset := 0
+	separators := strings.Count(root.path, "/")
+	trailingSeparator := root.path[len(root.path)-1] == '/'
+	if !trailingSeparator {
+		separators++
+	}
+	for range separators {
+		i := strings.IndexByte(path.path[offset:], '/')
+		if i < 0 {
+			panic("fswatch: folded path lost a directory boundary")
+		}
+		offset += i + 1
+	}
+	if trailingSeparator {
+		return path.path[offset:], true
+	}
+	return path.path[offset-1:], true
+}
+
+// The third result requests Unicode comparison; an ASCII rejection must not
+// reject an expanding alias just because the other spelling is ASCII.
+func pathSuffixASCII(root, path string) (string, bool, bool) {
+	i := 0
+	// Skip shared prefixes a word at a time, which is common when routing an
+	// event past sibling watches. String slice comparisons do not allocate.
+	for i+8 <= len(root) && i+8 <= len(path) && root[i:i+8] == path[i:i+8] {
+		i += 8
+	}
+	for ; i < len(root) && i < len(path); i++ {
+		a, b := root[i], path[i]
+		if a >= utf8.RuneSelf || b >= utf8.RuneSelf {
+			return "", false, true
+		}
+		if a == b {
+			continue
+		}
+		a |= 0x20
+		b |= 0x20
+		if a != b || a < 'a' || a > 'z' {
+			return "", false, false
+		}
+	}
+	if i == len(root) && (i == len(path) || path[i] == '/') {
+		return path[i:], true, false
+	}
+	return "", false, i < len(root) && root[i] >= utf8.RuneSelf || i < len(path) && path[i] >= utf8.RuneSelf
+}
+
+// Comparing the remaining components avoids assuming case-equivalent UTF-8
+// strings have the same byte length (for example, s and long s).
+func pathSuffixFoldUnicode(root, path string) (string, bool) {
+	for {
+		rootPart, rootRest, rootMore := strings.Cut(root, "/")
+		pathPart, pathRest, pathMore := strings.Cut(path, "/")
+		if !strings.EqualFold(rootPart, pathPart) {
+			return "", false
+		}
+		if !rootMore {
+			if pathMore {
+				return path[len(pathPart):], true
+			}
+			return "", true
+		}
+		if !pathMore {
+			return "", false
+		}
+		root, path = rootRest, pathRest
+	}
+}
+
+func (c pathComparer) contains(root, path string) bool {
+	_, ok := c.suffix(root, path)
+	return ok
+}
+
+func (c pathComparer) rebase(path, from, to string) (string, bool) {
+	p := comparisonPath{path: path}
+	return c.rebasePrepared(&p, comparisonPath{path: from}, to)
+}
+
+func (c pathComparer) rebasePrepared(path *comparisonPath, from comparisonPath, to string) (string, bool) {
+	if isInDirectoryOrSelf(from.path, path.path) {
+		return rebasePath(path.path, from.path, to), true
+	}
+	if !c.ignoreCase || from.path == "" {
+		return "", false
+	}
+	suffix, ok, unicode := pathSuffixASCII(from.path, path.path)
+	if unicode {
+		suffix, ok = c.suffixUnicode(from, path)
+	}
+	if !ok {
+		return "", false
+	}
+	return joinPathSuffix(to, suffix), true
+}

tsc/internal/fswatch/pathkey.go

@@ -0,0 +1,42 @@
+package fswatch
+
+import (
+	"strings"
+	"unicode/utf8"
+)
+
+// NativePathComparisonAvailable reports whether this platform implements the
+// native watch-name comparison. Volume sensitivity is queried separately.
+const NativePathComparisonAvailable = nativePathFolding
+
+// PathComparer describes the filename equivalence of a watched volume. Its zero
+// value compares bytes exactly. It is immutable and safe to share.
+type PathComparer struct {
+	comparer pathComparer
+}
+
+// Key returns a watch-only comparison key, not a filesystem path or a compiler
+// identity. Native Darwin comparers use the same CoreFoundation folding as the
+// watcher. Other comparers preserve bytes, including malformed UTF-8.
+func (c PathComparer) Key(path string) string {
+	if !c.comparer.ignoreCase || !nativePathFolding {
+		return path
+	}
+	if strings.IndexByte(path, 0) >= 0 {
+		return path
+	}
+	if folded := foldNativePath(path); folded != "" {
+		return folded
+	}
+	// Invalid UTF-8 and NUL-containing names are opaque, not Unicode aliases.
+	return path
+}
+
+// Rebase replaces a matching directory prefix while preserving the spelling and
+// byte boundaries of the remaining event path.
+func (c PathComparer) Rebase(path, from, to string) (string, bool) {
+	if !utf8.ValidString(path) || !utf8.ValidString(from) || strings.IndexByte(path, 0) >= 0 || strings.IndexByte(from, 0) >= 0 {
+		return (pathComparer{}).rebase(path, from, to)
+	}
+	return c.comparer.rebase(path, from, to)
+}

tsc/internal/fswatch/watcher.go

@@ -119,6 +119,17 @@ type WatchDirectoryRequest struct {
 type watchOptions struct {
 	ignore    func(path string) bool
 	recursive bool
+	file      string
+}
+
+// fileOption defers the file filter until the parent directory's comparer is
+// available, so WatchFile does not need a second filesystem query.
+type fileOption struct {
+	path string
+}
+
+func (o fileOption) applyWatchOption(opts *watchOptions) {
+	opts.file = o.path
 }
 
 type ignoreOption struct {
@@ -371,10 +382,10 @@ func (w *watcher) keyForDirWatch(dir string, recursive bool) string {
 	return dir
 }
 
-func (w *watcher) findCoveringRecursiveWatchLocked(dir string, physicalDir string) *dirWatch {
+func (w *watcher) findCoveringRecursiveWatchLocked(dir string, physicalDir string, comparer pathComparer) *dirWatch {
 	var best *dirWatch
 	for _, dw := range w.dirWatches {
-		if !dw.recursive || !isInDirectoryOrSelf(dw.dir, dir) || !isInDirectoryOrSelf(dw.physicalDir, physicalDir) {
+		if !dw.recursive || dw.comparer != comparer || !isInDirectoryOrSelf(dw.dir, dir) || !isInDirectoryOrSelf(dw.physicalDir, physicalDir) {
 			continue
 		}
 		if best == nil || len(dw.dir) > len(best.dir) {
@@ -416,7 +427,7 @@ func (w *watcher) findConsolidationDirLocked(dir string, physicalDir string) str
 	return ""
 }
 
-func (w *watcher) getOrCreateDirWatch(dir string, physicalDir string, recursive bool) *dirWatch {
+func (w *watcher) getOrCreateDirWatch(dir string, physicalDir string, recursive bool, comparer pathComparer) (*dirWatch, error) {
 	w.mu.Lock()
 	defer w.mu.Unlock()
 	if w.dirWatches == nil {
@@ -427,28 +438,35 @@ func (w *watcher) getOrCreateDirWatch(dir string, physicalDir string, recursive
 	}
 
 	if w.canShareRecursiveDirWatches() {
-		if dw := w.findCoveringRecursiveWatchLocked(dir, physicalDir); dw != nil {
-			return dw
+		if dw := w.findCoveringRecursiveWatchLocked(dir, physicalDir, comparer); dw != nil {
+			return dw, nil
 		}
 		if consolidationDir := w.findConsolidationDirLocked(dir, physicalDir); consolidationDir != "" {
-			dir = consolidationDir
-			physicalDir = physicalDirFor(dir)
-			recursive = true
-			if dw := w.findCoveringRecursiveWatchLocked(dir, physicalDir); dw != nil {
-				return dw
+			parentComparer, err := w.pathComparer(consolidationDir)
+			if err != nil {
+				return nil, err
+			}
+			if parentComparer == comparer {
+				dir = consolidationDir
+				physicalDir = physicalDirFor(dir)
+				recursive = true
+				if dw := w.findCoveringRecursiveWatchLocked(dir, physicalDir, comparer); dw != nil {
+					return dw, nil
+				}
 			}
 		}
 	}
 
 	key := w.keyForDirWatch(dir, recursive)
 	if dw, ok := w.dirWatches[key]; ok {
-		return dw
+		return dw, nil
 	}
 	dw := newDirWatch(dir, physicalDir, w.debounce)
+	dw.setComparer(comparer)
 	dw.sequence = w.sequence
 	dw.recursive = recursive
 	w.dirWatches[key] = dw
-	return dw
+	return dw, nil
 }
 
 func (w *watcher) removeDirWatch(dw *dirWatch) {
@@ -524,8 +542,17 @@ func (w *watcher) WatchDirectories(requests []WatchDirectoryRequest) ([]Watch, e
 			o.applyWatchOption(&sopts)
 		}
 
-		dw := w.getOrCreateDirWatch(dir, physicalDir, sopts.recursive)
-		id, _ := dw.watch(dir, physicalDir, sopts.recursive, fn, sopts.ignore)
+		comparer, err := w.pathComparer(dir)
+		if err != nil {
+			rollback()
+			return nil, err
+		}
+		dw, err := w.getOrCreateDirWatch(dir, physicalDir, sopts.recursive, comparer)
+		if err != nil {
+			rollback()
+			return nil, err
+		}
+		id, _ := dw.addCallback(dir, physicalDir, sopts.recursive, fn, sopts.ignore, sopts.file)
 		prepared = append(prepared, preparedWatch{dw: dw, id: id, recursive: sopts.recursive, dir: dir})
 		if _, ok := seenDirWatches[dw]; !ok {
 			seenDirWatches[dw] = struct{}{}
@@ -578,25 +605,7 @@ func (w *watcher) WatchFile(path string, fn WatchCallback) (Watch, error) {
 		return nil, errRootPath
 	}
 
-	return w.WatchDirectory(dir, fileCallback(path, fn))
-}
-
-// fileCallback wraps a WatchCallback so it only sees events for the
-// specific target path. Errors are always forwarded (with any matching
-// events delivered alongside) so callers don't lose overflow signals
-// just because their target wasn't in the same batch.
-func fileCallback(target string, fn WatchCallback) WatchCallback {
-	return func(events []Event, err error) {
-		var filtered []Event
-		for _, e := range events {
-			if e.Path == target {
-				filtered = append(filtered, e)
-			}
-		}
-		if len(filtered) > 0 || err != nil {
-			fn(filtered, err)
-		}
-	}
+	return w.WatchDirectory(dir, fn, fileOption{path: path})
 }
 
 type watch struct {
@@ -771,17 +780,21 @@ func (b *watcherBase) handleWatcherError(werr *dirWatchError) {
 // ----- dirWatch: per-directory watch state -------------------------
 
 type callback struct {
-	id               uint64
-	dir              string
-	physicalDir      string
-	watchDir         string
-	watchPhysicalDir string
-	recursive        bool
-	fn               WatchCallback
-	ignore           func(path string) bool
-	sinceSeq         uint64
-	terminal         error
-	delivered        bool
+	id                 uint64
+	dir                string
+	physicalDir        string
+	watchDir           string
+	watchPhysicalDir   string
+	recursive          bool
+	fn                 WatchCallback
+	ignore             func(path string) bool
+	sinceSeq           uint64
+	terminal           error
+	delivered          bool
+	comparer           pathComparer
+	dirComparison      comparisonPath
+	physicalComparison comparisonPath
+	fileComparison     comparisonPath
 }
 
 // dirWatchError associates an error with a specific directory watch.
@@ -800,9 +813,12 @@ type dirWatch struct {
 	dir string
 	// physicalDir is the path passed to OS watcher APIs. It differs from dir
 	// when dir or an ancestor is a symlink or reparse point to a directory.
-	physicalDir string
-	recursive   bool
-	events      eventList
+	physicalDir     string
+	recursive       bool
+	events          eventList
+	comparer        pathComparer
+	dirFold         string
+	physicalDirFold string
 
 	// state stores per-directory platform-specific bookkeeping (fsevents, windows).
 	state any
@@ -822,6 +838,16 @@ func newDirWatch(dir string, physicalDir string, db *debounce) *dirWatch {
 	return dw
 }
 
+func (dw *dirWatch) setComparer(comparer pathComparer) {
+	dw.comparer = comparer
+	dw.dirFold = comparer.prepare(dw.dir).folded
+	if dw.physicalDir == dw.dir {
+		dw.physicalDirFold = dw.dirFold
+	} else {
+		dw.physicalDirFold = comparer.prepare(dw.physicalDir).folded
+	}
+}
+
 // physicalDirFor returns the physical path to watch for dir. If dir, or an
 // ancestor of dir, is a symlink or reparse point, events are subscribed on its
 // realpath while callbacks still use dir.
@@ -968,12 +994,20 @@ func (dw *dirWatch) triggerCallbacks() {
 	}
 	dw.mu.Unlock()
 
+	var comparisons comparisonCache
 	for i, cb := range cbs {
 		cbEvents := eventsByCallback[i]
-		if cb.ignore != nil || !cb.recursive || cb.dir != dw.dir {
+		if cb.ignore != nil || !cb.recursive || cb.dir != dw.dir || cb.fileComparison.path != "" {
 			filtered := make([]Event, 0, len(cbEvents))
 			for _, e := range cbEvents {
-				e = cb.mapEvent(e)
+				e = cb.mapEventCached(e, &comparisons)
+				if cb.fileComparison.path != "" {
+					path := comparisonPath{path: e.Path, cache: &comparisons}
+					if suffix, ok := cb.comparer.suffixPrepared(cb.fileComparison, &path); !ok || suffix != "" {
+						continue
+					}
+					e.Path = cb.fileComparison.path
+				}
 				if cb.ignore != nil && cb.ignore(e.Path) {
 					continue
 				}
@@ -1002,10 +1036,18 @@ func (dw *dirWatch) triggerCallbacks() {
 }
 
 func (cb callback) mapEvent(e Event) Event {
-	if cb.physicalDir != "" && cb.physicalDir != cb.dir {
-		physicalPath := cb.eventPhysicalPath(e.Path)
-		if isInDirectoryOrSelf(cb.physicalDir, physicalPath) {
-			e.Path = rebasePath(physicalPath, cb.physicalDir, cb.dir)
+	return cb.mapEventCached(e, nil)
+}
+
+func (cb callback) mapEventCached(e Event, cache *comparisonCache) Event {
+	if cb.physicalDir != "" && (cb.physicalDir != cb.dir || cb.comparer.ignoreCase) {
+		physicalPath := comparisonPath{path: cb.eventPhysicalPath(e.Path), cache: cache}
+		root := cb.physicalComparison
+		if root.path == "" {
+			root.path = cb.physicalDir
+		}
+		if path, ok := cb.comparer.rebasePrepared(&physicalPath, root, cb.dir); ok {
+			e.Path = path
 		}
 	}
 	return e
@@ -1022,13 +1064,18 @@ func (dw *dirWatch) terminateCallbacksForDeletedRoot(path string, seq uint64, er
 	dw.mu.Lock()
 	defer dw.mu.Unlock()
 	changed := false
+	var comparisons comparisonCache
+	deleted := comparisonPath{path: path, cache: &comparisons}
 	for i := range dw.callbacks {
 		cb := &dw.callbacks[i]
 		if cb.delivered || cb.terminal != nil || cb.sinceSeq >= seq {
 			continue
 		}
-		physicalPath := cb.eventPhysicalPath(path)
-		if isInDirectoryOrSelf(path, cb.dir) || (cb.physicalDir != cb.dir && isInDirectoryOrSelf(physicalPath, cb.physicalDir)) {
+		physicalPath := comparisonPath{path: cb.eventPhysicalPath(path), cache: &comparisons}
+		dir, physical := cb.dirComparison, cb.physicalComparison
+		_, logicalMatch := cb.comparer.suffixPrepared(deleted, &dir)
+		_, physicalMatch := cb.comparer.suffixPrepared(physicalPath, &physical)
+		if logicalMatch || physicalMatch {
 			cb.terminal = err
 			changed = true
 		}
@@ -1074,6 +1121,10 @@ func isDirectChild(dir, path string) bool {
 }
 
 func (dw *dirWatch) watch(dir string, physicalDir string, recursive bool, fn WatchCallback, ignore func(path string) bool) (uint64, bool) {
+	return dw.addCallback(dir, physicalDir, recursive, fn, ignore, "")
+}
+
+func (dw *dirWatch) addCallback(dir string, physicalDir string, recursive bool, fn WatchCallback, ignore func(path string) bool, file string) (uint64, bool) {
 	dw.mu.Lock()
 	defer dw.mu.Unlock()
 	dw.nextCBID++
@@ -1082,7 +1133,12 @@ func (dw *dirWatch) watch(dir string, physicalDir string, recursive bool, fn Wat
 	if dw.sequence != nil {
 		sinceSeq = dw.sequence()
 	}
-	dw.callbacks = append(dw.callbacks, callback{id: id, dir: dir, physicalDir: physicalDir, watchDir: dw.dir, watchPhysicalDir: dw.physicalDir, recursive: recursive, fn: fn, ignore: ignore, sinceSeq: sinceSeq})
+	dw.callbacks = append(dw.callbacks, callback{
+		id: id, dir: dir, physicalDir: physicalDir, watchDir: dw.dir, watchPhysicalDir: dw.physicalDir,
+		recursive: recursive, fn: fn, ignore: ignore, sinceSeq: sinceSeq, comparer: dw.comparer,
+		dirComparison: dw.comparer.prepare(dir), physicalComparison: dw.comparer.prepare(physicalDir),
+		fileComparison: dw.comparer.prepare(file),
+	})
 	return id, true
 }
 

tsc/internal/fswatch/watcher_test.go

@@ -575,6 +575,130 @@ func containsEvent(got []Event, typ EventKind, path string) bool {
 	return false
 }
 
+func TestPathComparer(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		root       string
+		path       string
+		suffix     string
+		exact      bool
+		ignoreCase bool
+	}{
+		{"/root", "/root", "", true, true},
+		{"/root", "/root/file.ts", "/file.ts", true, true},
+		{"/root", "/ROOT", "", false, true},
+		{"/root", "/ROOT/File.ts", "/File.ts", false, true},
+		{"/root", "/ROOT/Nested/File.ts", "/Nested/File.ts", false, true},
+		{"/root", "/ROOT2/File.ts", "", false, false},
+		{"/root", "/roo", "", false, false},
+		{"/root/sub", "/ROOT", "", false, false},
+		{"/root", "/other/File.ts", "", false, false},
+		{"/root", "/ROOTish/File.ts", "", false, false},
+		{"/root/sub", "/ROOT/SUB", "", false, true},
+		{"/root/sub", "/ROOT/su", "", false, false},
+		{"/root/[", "/ROOT/{/File.ts", "", false, false},
+		{"/root/@", "/ROOT/`/File.ts", "", false, false},
+		{"/", "/File.ts", "File.ts", true, true},
+		{"/", "/", "", true, true},
+		{"", "", "", false, false},
+		{"", "/File.ts", "", false, false},
+		{"/caf\u00e9", "/CAF\u00c9/File.ts", "/File.ts", false, true},
+		{"/s", "/\u017f/File.ts", "/File.ts", false, true},
+		{"/\u017f", "/S/File.ts", "/File.ts", false, true},
+		{"/s/sub", "/\u017f/SUB/File.ts", "/File.ts", false, true},
+		{"/\u017f/sub", "/S/SUB/File.ts", "/File.ts", false, true},
+		{"/s", "/\u017foo/File.ts", "", false, false},
+		{"/k", "/\u212a/File.ts", "/File.ts", false, true},
+		{"/\u03c3", "/\u03c2/File.ts", "/File.ts", false, true},
+		{"/\u00e9", "/\u00c8/File.ts", "", false, false},
+		{"/\u00df", "/SS/File.ts", "/File.ts", false, nativePathFolding},
+		{"/root/s", "/ROOT/\u017f/File.ts", "/File.ts", false, true},
+		{"/root/\u017f", "/ROOT/S", "", false, true},
+	}
+	for _, tt := range tests {
+		for _, ignoreCase := range []bool{false, true} {
+			comparer := pathComparer{ignoreCase: ignoreCase}
+			want := tt.exact
+			if ignoreCase {
+				want = tt.ignoreCase
+			}
+			suffix, ok := comparer.suffix(tt.root, tt.path)
+			if ok != want || ok && suffix != tt.suffix {
+				t.Errorf("suffix(%q, %q), ignoreCase=%v: got (%q, %v), want (%q, %v)", tt.root, tt.path, ignoreCase, suffix, ok, tt.suffix, want)
+			}
+			if comparer.contains(tt.root, tt.path) != want {
+				t.Errorf("contains(%q, %q), ignoreCase=%v: want %v", tt.root, tt.path, ignoreCase, want)
+			}
+			for _, to := range []string{"/display", "/"} {
+				rebased, ok := comparer.rebase(tt.path, tt.root, to)
+				if ok != want || ok && rebased != joinPathSuffix(to, tt.suffix) {
+					t.Errorf("rebase(%q, %q, %q), ignoreCase=%v: got (%q, %v)", tt.path, tt.root, to, ignoreCase, rebased, ok)
+				}
+			}
+		}
+	}
+}
+
+func TestPathComparerUnicodeAlignment(t *testing.T) {
+	t.Parallel()
+	parts := []string{"s", "S", "\u017f", "k", "K", "\u212a", "\u03c3", "\u03c2", "\u00e9", "\u00c9", "\u00c8", "\U00010400", "\U00010428", "\xff", "\xfe", "\xc3"}
+	comparer := pathComparer{ignoreCase: true}
+	for padding := range 16 {
+		prefix := "/" + strings.Repeat("a", padding)
+		for _, a := range parts {
+			for _, b := range parts {
+				for _, child := range []string{"", "/child"} {
+					root := prefix + a + child
+					path := prefix + b + strings.ToUpper(child) + "/File.ts"
+					want := strings.EqualFold(a, b)
+					suffix, ok := comparer.suffix(root, path)
+					if ok != want || ok && suffix != "/File.ts" {
+						t.Fatalf("suffix(%q, %q): got (%q, %v), want match=%v", root, path, suffix, ok, want)
+					}
+				}
+			}
+		}
+	}
+}
+
+func TestFileCallbackCaseSensitivity(t *testing.T) {
+	t.Parallel()
+	for _, ignoreCase := range []bool{false, true} {
+		var got []Event
+		dw := newDirectWatcher(t, "/root")
+		dw.setComparer(pathComparer{ignoreCase: ignoreCase})
+		dw.addCallback("/root", "/root", false, func(events []Event, err error) {
+			got = append(got, events...)
+		}, nil, "/root/file.ts")
+		dw.events.update("/root/FILE.ts")
+		dw.events.update("/root/other.ts")
+		dw.triggerCallbacks()
+		if ignoreCase {
+			if len(got) != 1 || got[0].Path != "/root/file.ts" {
+				t.Fatalf("case-insensitive callback: got %v", got)
+			}
+		} else if len(got) != 0 {
+			t.Fatalf("case-sensitive callback: got %v", got)
+		}
+	}
+}
+
+func TestPathComparerExactKeys(t *testing.T) {
+	t.Parallel()
+	c := PathComparer{}
+	for _, path := range []string{"/A/File.ts", "/straße/İ.ts", "/cafe\u0301.ts", "/bad\xff.ts"} {
+		if got := c.Key(path); got != path {
+			t.Fatalf("Key(%q) = %q", path, got)
+		}
+	}
+	if _, ok := c.Rebase("/A/file.ts", "/a", "/target"); ok {
+		t.Fatal("zero comparer must use exact matching")
+	}
+	if got, ok := c.Rebase("/a/file.ts", "/a", "/target"); !ok || got != "/target/file.ts" {
+		t.Fatalf("Rebase = %q, %v", got, ok)
+	}
+}
+
 func TestRebasePath(t *testing.T) {
 	t.Parallel()
 
@@ -2107,9 +2231,23 @@ func TestFileCallbackForwardsErrAlongsideEvents(t *testing.T) {
 		err    error
 	}
 	var got []call
-	cb := fileCallback(target, func(events []Event, err error) {
+	dw := newDirectWatcher(t, "/abs/dir")
+	dw.addCallback("/abs/dir", "/abs/dir", false, func(events []Event, err error) {
 		got = append(got, call{events: events, err: err})
-	})
+	}, nil, target)
+	cb := func(events []Event, err error) {
+		for _, e := range events {
+			if e.Kind == EventDelete {
+				dw.events.remove(e.Path)
+			} else {
+				dw.events.update(e.Path)
+			}
+		}
+		if err != nil {
+			dw.events.setError(err)
+		}
+		dw.triggerCallbacks()
+	}
 
 	// Plain events: only target events pass through, sibling dropped.
 	cb([]Event{{Kind: EventUpdate, Path: target}, {Kind: EventUpdate, Path: other}}, nil)

tsc/testdata/baselines/reference/tsbuildWatch/dependencyUpdate/watches-absolute-non-root-dependency-updates.js

@@ -133,10 +133,10 @@ export const value = myValue;
 
 Watch Registrations::
 Directory watches::
-  c:/home/src/tslibs/ts/lib
-  c:/work/project
-  c:/work/project/src (recursive)
-  d:/work/deps
+  C:/home/src/tslibs/TS/Lib
+  C:/work/project
+  C:/work/project/src (recursive)
+  D:/work/deps
 tsconfig.json::
 SemanticDiagnostics::
 *refresh*    C:/home/src/tslibs/TS/Lib/lib.es2025.full.d.ts
@@ -257,10 +257,10 @@ Output::
 
 Watch Registrations::
 Directory watches::
-  c:/home/src/tslibs/ts/lib
-  c:/work/project
-  c:/work/project/src (recursive)
-  d:/work/deps
+  C:/home/src/tslibs/TS/Lib
+  C:/work/project
+  C:/work/project/src (recursive)
+  D:/work/deps
 tsconfig.json::
 SemanticDiagnostics::
 *refresh*    D:/work/deps/dep.d.ts