Improve copying current glyphs when building frame matrix

* src/dispnew.c (build_frame_matrix_from_leaf_window): Don't make space
glyphs. More comments.

1 files changed, 15 insertions, 10 deletions

diff --git a/src/dispnew.c b/src/dispnew.c
index 302f7db8815..23ef5ff699b 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -2627,22 +2627,27 @@ build_frame_matrix_from_leaf_window (struct glyph_matrix *frame_matrix, struct w
           current_row_p = 1;
         }
 
+      /* If someone asks why we are copying current glyphs here, and
+         maybe never enable the desired frame row we copy to:
+
+         - there might be a window to the right of this one that has a
+           corresponding desired window row.
+         - we need the complete frame row for scrolling.  */
       if (current_row_p)
         {
-          /* If the desired glyphs for this row haven't been built,
-             copy from the corresponding current row, but only if it
-             is enabled, because ottherwise its contents are invalid.  */
+          /* If the desired glyphs for this row haven't been built, copy
+             from the corresponding current row. If that row is not
+             enabled, its contents might be invalid.  Make sure that
+             glyphs have valid frames set in that case.  This is closer
+             to what we did before child frames were added, and seems to
+             be something tty redisplay implicitly relies on.  */
           struct glyph *to = frame_row->glyphs[TEXT_AREA] + window_matrix->matrix_x;
           struct glyph *from = window_row->glyphs[0];
           for (int i = 0; i < window_matrix->matrix_w; ++i)
             {
-              if (window_row->enabled_p)
-                to[i] = from[i];
-              else
-                {
-                  to[i] = space_glyph;
-                  to[i].frame = f;
-                }
+              to[i] = from[i];
+              if (!window_row->enabled_p)
+                to[i].frame = f;
             }
         }
       else