scratch/tty-child-frames 4182dfb7e84 1/2: ; Clarify a comment in dispnew.c

[Eli Zaretskii]

branch: scratch/tty-child-frames
commit 4182dfb7e844bcc7148a3d7f1084cef6982b36d9
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    ; Clarify a comment in dispnew.c
---
 src/dispnew.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/dispnew.c b/src/dispnew.c
index 200ffaaca21..9caeb9d1694 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -1176,9 +1176,12 @@ line_hash_code (struct frame *f, struct glyph_row *row)
        {
          int c = glyph->u.ch;
          int face_id = glyph->face_id;
-         /* Struct frame can move with igc, and so on.  But we need
-            something that takes different frames into account.  Use the
-            face_cache pointer for that which is malloc'd.  */
+         /* A given row of a frame glyph matrix could have glyphs
+            from more than one frame, if child frames are displayed.
+            Since face_id of a face depends on the frame (it's an
+            index into the frame's face cache), we need the hash
+            value to include something specific to the frame, and we
+            use the frame cache's address for that purpose.  */
          if (glyph->frame && glyph->frame != f)
            face_id += (ptrdiff_t) glyph->frame->face_cache;
          if (FRAME_MUST_WRITE_SPACES (f))