Fix start-of-box-face display after display and overlay strings

* src/xdisp.c (pop_it): Recompute the 'start_of_box_run_p' flag,
like we do in 'handle_fontified_prop', when faces change while
iterating over a buffer or string.  (Bug#51550)

1 files changed, 23 insertions, 4 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index a6c122aee8b..6c63278f528 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -6841,6 +6841,7 @@ pop_it (struct it *it)
   struct iterator_stack_entry *p;
   bool from_display_prop = it->from_disp_prop_p;
   ptrdiff_t prev_pos = IT_CHARPOS (*it);
+  int prev_face_id = it->face_id;
 
   eassert (it->sp > 0);
   --it->sp;
@@ -6876,9 +6877,18 @@ pop_it (struct it *it)
 
         /* Restore the face_box_p flag, since it could have been
            overwritten by the face of the object that we just finished
-           displaying.  */
+           displaying.  Also, set the start_of_box_run_p flag if the
+           change in faces requires that.  */
         if (face)
-          it->face_box_p = face->box != FACE_NO_BOX;
+          {
+            struct face *prev_face = FACE_FROM_ID_OR_NULL (it->f, prev_face_id);
+
+            if (!(it->start_of_box_run_p && prev_face && prev_face->box))
+              it->start_of_box_run_p = (face->box != FACE_NO_BOX
+                                        && (prev_face == NULL
+                                            || prev_face->box == FACE_NO_BOX));
+            it->face_box_p = face->box != FACE_NO_BOX;
+          }
         it->object = it->w->contents;
       }
       break;
@@ -6888,9 +6898,18 @@ pop_it (struct it *it)
 
         /* Restore the face_box_p flag, since it could have been
            overwritten by the face of the object that we just finished
-           displaying.  */
+           displaying.  Also, set the start_of_box_run_p flag if the
+           change in faces requires that.  */
         if (face)
-          it->face_box_p = face->box != FACE_NO_BOX;
+          {
+            struct face *prev_face = FACE_FROM_ID_OR_NULL (it->f, prev_face_id);
+
+            if (!(it->start_of_box_run_p && prev_face && prev_face->box))
+              it->start_of_box_run_p = (face->box != FACE_NO_BOX
+                                        && (prev_face == NULL
+                                            || prev_face->box == FACE_NO_BOX));
+            it->face_box_p = face->box != FACE_NO_BOX;
+          }
         it->object = it->string;
       }
       break;