; Clarify a comment in dispnew.c

1 files 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))