patch 9.2.0964: 'isprint' is applied to the leading byte of a character

Problem:  Excluding a value with 'isprint' also breaks the display of
          unrelated characters whose leading byte has that value.  After
          ":set isprint+=^226" the character U+222B is displayed as "<"
          (Emilien Breton).
Solution: Apply 'isprint' to the character value only, not to the leading
          byte of its UTF-8 encoding (Hirohito Higashi).

fixes:  #21053
closes: #21055

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/drawline.c

@@ -3325,8 +3325,9 @@ win_line(
 		}
 	    }
 
-	    // Handling of non-printable characters.
-	    if (!vim_isprintc(c))
+	    // Handling of non-printable characters. 'isprint' does not apply
+	    // to a UTF-8 leading byte, the character was checked above.
+	    if (!vim_isprintc(c) && !(enc_utf8 && c >= 0x80))
 	    {
 		// when getting a character from the file, we may have to
 		// turn it into something else on the way to putting it

src/testdir/test_utf8.vim

@@ -388,4 +388,22 @@ func Test_overlong_utf8_cells()
   bwipe!
 endfunc
 
+func Test_isprint_leading_byte()
+  new
+  let save_isprint = &isprint
+  " 226 is the leading byte of U+222B, which must not be affected.
+  set isprint+=^226
+  call setline(1, "∫")
+  redraw
+  call assert_equal("∫", ScreenLines(1, 1)[0])
+
+  " U+00E2 has the character value 226 and is still excluded.
+  call setline(1, "â")
+  redraw
+  call assert_equal('<e2>', ScreenLines(1, 4)[0])
+
+  let &isprint = save_isprint
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -763,6 +763,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    964,
 /**/
     963,
 /**/