Fix display of mouse-highlight produced by overlapping overlays

* src/xfaces.c (face_at_buffer_position): If called to find the
mouse-face, only consider the highest-priority source for that
face, and ignore the rest.  Previously, all the mouse-face
definitions at POS were merged in that case.
* src/xdisp.c (note_mouse_highlight): Record the overlay that
specifies mouse-face _after_ clearing the info about the previous
overlay, so as not to clear the information about the just-recorded
overlay.  (Bug#25906)

2 files changed, 45 insertions, 12 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 1f8878408be..851a32b4f88 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -30439,12 +30439,14 @@ note_mouse_highlight (struct frame *f, int x, int y)
              no need to do that again.  */
           if (!NILP (overlay) && EQ (overlay, hlinfo->mouse_face_overlay))
             goto check_help_echo;
-          hlinfo->mouse_face_overlay = overlay;
 
           /* Clear the display of the old active region, if any.  */
           if (clear_mouse_face (hlinfo))
             cursor = No_Cursor;
 
+          /* Record the overlay, if any, to be highlighted.  */
+          hlinfo->mouse_face_overlay = overlay;
+
           /* If no overlay applies, get a text property.  */
           if (NILP (overlay))
             mouse_face = Fget_text_property (position, Qmouse_face, object);
diff --git a/src/xfaces.c b/src/xfaces.c
index b5dbb53ca20..7fcaef4e41a 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -5869,7 +5869,10 @@ compute_char_face (struct frame *f, int ch, Lisp_Object prop)
    LIMIT is a position not to scan beyond.  That is to limit the time
    this function can take.
 
-   If MOUSE, use the character's mouse-face, not its face.
+   If MOUSE, use the character's mouse-face, not its face, and only
+   consider the highest-priority source of mouse-face at POS,
+   i.e. don't merge different mouse-face values if more than one
+   source specifies it.
 
    BASE_FACE_ID, if non-negative, specifies a base face id to use
    instead of DEFAULT_FACE_ID.
@@ -5949,19 +5952,47 @@ face_at_buffer_position (struct window *w, ptrdiff_t pos,
 
   /* Now merge the overlay data.  */
   noverlays = sort_overlays (overlay_vec, noverlays, w);
-  for (i = 0; i < noverlays; i++)
+  /* For mouse-face, we need only the single highest-priority face
+     from the overlays, if any.  */
+  if (mouse)
     {
-      Lisp_Object oend;
-      ptrdiff_t oendpos;
+      for (prop = Qnil, i = noverlays - 1; i >= 0 && NILP (prop); --i)
+        {
+          Lisp_Object oend;
+          ptrdiff_t oendpos;
 
-      prop = Foverlay_get (overlay_vec[i], propname);
-      if (!NILP (prop))
-        merge_face_ref (f, prop, attrs, true, 0);
+          prop = Foverlay_get (overlay_vec[i], propname);
+          if (!NILP (prop))
+            {
+              /* Overlays always take priority over text properties,
+                 so discard the mouse-face text property, if any, and
+                 use the overlay property instead.  */
+              memcpy (attrs, default_face->lface, sizeof attrs);
+              merge_face_ref (f, prop, attrs, true, 0);
+            }
 
-      oend = OVERLAY_END (overlay_vec[i]);
-      oendpos = OVERLAY_POSITION (oend);
-      if (oendpos < endpos)
-        endpos = oendpos;
+          oend = OVERLAY_END (overlay_vec[i]);
+          oendpos = OVERLAY_POSITION (oend);
+          if (oendpos < endpos)
+            endpos = oendpos;
+        }
+    }
+  else
+    {
+      for (i = 0; i < noverlays; i++)
+        {
+          Lisp_Object oend;
+          ptrdiff_t oendpos;
+
+          prop = Foverlay_get (overlay_vec[i], propname);
+          if (!NILP (prop))
+            merge_face_ref (f, prop, attrs, true, 0);
+
+          oend = OVERLAY_END (overlay_vec[i]);
+          oendpos = OVERLAY_POSITION (oend);
+          if (oendpos < endpos)
+            endpos = oendpos;
+        }
     }
 
   *endptr = endpos;