emacs-27 daff3bd: Avoid crashes when a reversed glyph row starts with a composition

[Eli Zaretskii]

branch: emacs-27
commit daff3bda10d15fe20f5f6e9c5f5ca60b97cf80df
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Avoid crashes when a reversed glyph row starts with a composition
    
    * src/dispnew.c (build_frame_matrix_from_leaf_window): Add an
    assertion to prevent us from overwriting non-char glyphs with the
    vertical border glyph.
    * src/xdisp.c (extend_face_to_end_of_line): Account for one glyph
    possibly inserted by append_space_for_newline.  (Bug#44506)
    Remove a kludgey correction for an off-by-one error in column
    counting, which is no longer needed.
---
 src/dispnew.c | 10 +++++++---
 src/xdisp.c   | 15 ++++++++-------
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/src/dispnew.c b/src/dispnew.c
index df55b32..7822829 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -2559,11 +2559,15 @@ build_frame_matrix_from_leaf_window (struct 
glyph_matrix *frame_matrix, struct w
             the corresponding frame row to be updated.  */
          frame_row->enabled_p = true;
 
-          /* Maybe insert a vertical border between horizontally adjacent
+         /* Maybe insert a vertical border between horizontally adjacent
             windows.  */
-          if (GLYPH_CHAR (right_border_glyph) != 0)
+         if (GLYPH_CHAR (right_border_glyph) != 0)
            {
-              struct glyph *border = window_row->glyphs[LAST_AREA] - 1;
+             struct glyph *border = window_row->glyphs[LAST_AREA] - 1;
+             /* It's a subtle bug if we are overwriting some non-char
+                glyph with the vertical border glyph.  */
+             eassert (border->type == CHAR_GLYPH);
+             border->type = CHAR_GLYPH;
              SET_CHAR_GLYPH_FROM_GLYPH (*border, right_border_glyph);
            }
 
diff --git a/src/xdisp.c b/src/xdisp.c
index 71a5f1c..681df09 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -22074,13 +22074,14 @@ extend_face_to_end_of_line (struct it *it)
                      default_face->id : face->id);
 
       /* Display fill-column indicator if needed.  */
-      /* We need to subtract 1 to the indicator_column here because we
-        will add the indicator IN the column indicator number, not
-        after it.  We compare the variable it->current_x before
-        producing the glyph.  When FRAME_WINDOW_P we subtract
-        CHAR_WIDTH calculating STRETCH_WIDTH for the same reason.  */
-      const int indicator_column =
-       fill_column_indicator_column (it, 1) - 1;
+      const int indicator_column = fill_column_indicator_column (it, 1);
+
+      /* Make sure our idea of current_x is in sync with the glyphs
+        actually in the glyph row.  They might differ because
+        append_space_for_newline can insert one glyph without
+        updating current_x.  */
+      it->current_x = it->glyph_row->used[TEXT_AREA];
+
       do
        {
          if (it->current_x != indicator_column)