[gedit] Window: fullscreen mode: fix headerbar shown/hidden "stuttering"

[Sébastien Wilmet <swilmet src gnome org>]

commit 8d3e6faf2b1d523526516944abe63cd89acf4989
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Wed Nov 27 12:47:08 2019 +0100

    Window: fullscreen mode: fix headerbar shown/hidden "stuttering"

 gedit/gedit-window.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)
---
diff --git a/gedit/gedit-window.c b/gedit/gedit-window.c
index 96a127b21..fb1f6ab8d 100644
--- a/gedit/gedit-window.c
+++ b/gedit/gedit-window.c
@@ -1755,6 +1755,34 @@ on_fullscreen_eventbox_leave_notify_event (GtkWidget        *fullscreen_eventbox
                                           GdkEventCrossing *event,
                                           GeditWindow      *window)
 {
+       if (-1.0 <= event->y && event->y <= 0.0)
+       {
+               /* Ignore the event.
+                *
+                * Leave notify events are received with -1 <= y <= 0
+                * coordinates, although the GeditWindow is in fullscreen mode
+                * and when there are no screens above (it's maybe a bug in an
+                * underlying library).
+                * If we hide the headerbar when those events happen, then it
+                * makes the headerbar to be shown/hidden a lot of time in a
+                * short period of time, i.e. a "stuttering". In other words
+                * lots of leave/enter events are received when moving the mouse
+                * upwards on the screen when the mouse is already at the top.
+                * The expected leave event has a positive event->y value being
+                * >= to the height of the headerbar (approximately
+                * 40 <= y <= 50). So clearly when we receive a leave event with
+                * event->y <= 0, it means that the mouse has left the eventbox
+                * on the wrong side.
+                * The -1.0 <= event->y is there (instead of just <= 0.0) in the
+                * case that there is another screen *above*, even if this
+                * heuristic/workaround is not perfect in that case. But that
+                * case is quite rare, so it's probably a good enough solution.
+                *
+                * If you see a better solution...
+                */
+               return GDK_EVENT_PROPAGATE;
+       }
+
        window->priv->in_fullscreen_eventbox = FALSE;
        update_fullscreen_revealer_state (window);