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.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/dispextern.h b/src/dispextern.h
index 2f55d8cbc87..430afbf09a3 100644
--- a/src/dispextern.h
+++ b/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[];