Fix display of line numbers in GTK builds

* src/xdisp.c (should_produce_line_number) [USE_GTK]: Make sure
tip_frame is indeed a tooltip frame, before disabling line numbers
on it.  (Bug#27647)

* src/dispextern.h (tip_frame): Add commentary describing the
kludgey usage of this variable in GTK builds.

src/dispextern.h

@@ -3452,7 +3452,14 @@ void gamma_correct (struct frame *, COLORREF *);
 void x_implicitly_set_name (struct frame *, Lisp_Object, Lisp_Object);
 void x_change_tool_bar_height (struct frame *f, int);
 
+/* The frame used to display a tooltip.
+
+   Note: In a GTK build with non-zero x_gtk_use_system_tooltips, this
+   variable holds the frame that shows the tooltip, not the frame of
+   the tooltip itself, so checking whether a frame is a tooltip frame
+   cannot just compare the frame to what this variable holds.  */
 extern Lisp_Object tip_frame;
+
 extern Window tip_window;
 extern frame_parm_handler x_frame_parm_handlers[];
 

src/xdisp.c

@@ -21132,7 +21132,13 @@ should_produce_line_number (struct it *it)
 
 #ifdef HAVE_WINDOW_SYSTEM
   /* Don't display line number in tooltip frames.  */
-  if (FRAMEP (tip_frame) && EQ (WINDOW_FRAME (it->w), tip_frame))
+  if (FRAMEP (tip_frame) && EQ (WINDOW_FRAME (it->w), tip_frame)
+#ifdef USE_GTK
+      /* GTK builds store in tip_frame the frame that shows the tip,
+	 so we need an additional test.  */
+      && !NILP (Fframe_parameter (tip_frame, Qtooltip))
+#endif
+      )
     return false;
 #endif