diff --git a/src/xdisp.c b/src/xdisp.c
index 97ba6721781..436153b2bae 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -13861,12 +13861,17 @@ note_tab_bar_highlight (struct frame *f, int x, int y)
 
   bool mouse_down_p = false;
 #ifndef HAVE_NS
-  /* Mouse is down, but on different tab-bar item?  */
+  /* Mouse is down, but on different tab-bar item?  Or alternatively,
+     the mouse might've been pressed somewhere we don't know about,
+     and then have moved onto the tab bar.  In this case,
+     last_tab_bar_item is -1, so we DTRT and behave like other
+     programs by displaying the item as sunken. */
   Display_Info *dpyinfo = FRAME_DISPLAY_INFO (f);
   mouse_down_p = (gui_mouse_grabbed (dpyinfo)
 		  && f == dpyinfo->last_mouse_frame);
 
-  if (mouse_down_p && f->last_tab_bar_item != prop_idx)
+  if (mouse_down_p && f->last_tab_bar_item != prop_idx
+      && f->last_tab_bar_item != -1)
     return;
 #endif
   draw = mouse_down_p ? DRAW_IMAGE_SUNKEN : DRAW_IMAGE_RAISED;