bug#34680: Display monitor frames not warmed up

[Juri Linkov]

>> I guess calling the function ‘message’ before calling
>> ‘display-monitor-attributes-list’ performs some redisplay that adds
>> the frame to the list of frames in ‘display-monitor-attributes-list’.
>
> It also fixes the issue when using any of the following
> ‘redisplay’, ‘redraw-display’, ‘redraw-frame’, or just
> ‘display-monitor-attributes-list’ before calling
> ‘frame-monitor-attributes’ for the first time.
>
> Just calling ‘display-monitor-attributes-list’ somehow “registers” the
> frame in the list of frames, so the next call of 
> ‘display-monitor-attributes-list’
> returns the attribute ‘frames’ containing the frame.

I can't explain this, but this is the minimal patch that fixes my
problem.  This dry run by calling gdk_display_get_monitor_at_window
ignores its first wrong return value, so its subsequent calls return
correct values.

diff --git a/src/xfns.c b/src/xfns.c
index a627b7e19e..9096595625 100644
--- a/src/xfns.c
+++ b/src/xfns.c
@@ -4943,6 +4943,7 @@ Internal use only, use `display-monitor-attributes-list' 
instead.  */)
          && !FRAME_TOOLTIP_P (f))
        {
          GdkWindow *gwin = gtk_widget_get_window (FRAME_GTK_WIDGET (f));
+         gdk_display_get_monitor_at_window (gdpy, gwin);
 
 #if GTK_CHECK_VERSION (3, 22, 0)
           for (i = 0; i < n_monitors; i++)