[mutter] window: Don't override tile monitor

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 64ced1632e277e4fc0b1f4de3f5bf229c6cf885b
Author: Florian Müllner <fmuellner gnome org>
Date:   Mon Oct 12 14:08:28 2020 +0200

    window: Don't override tile monitor
    
    Commit 033f0d11bf added a fallback in case the tile monitor wasn't
    set before, but didn't actually check for a previously set value.
    As a result, the "fallback" is not set unconditionally, which may
    differ from the expected monitor: The tile monitor is determined
    by the pointer position, while the window's monitor is the one
    where the biggest part of the window resides on.
    
    https://gitlab.gnome.org/GNOME/mutter/-/issues/1389

 src/core/window.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/core/window.c b/src/core/window.c
index dd48820819..ce6b01bfdf 100644
--- a/src/core/window.c
+++ b/src/core/window.c
@@ -3158,7 +3158,7 @@ meta_window_tile (MetaWindow   *window,
       window->tile_monitor_number = -1;
       return;
     }
-  else
+  else if (window->tile_monitor_number < 0)
     {
       window->tile_monitor_number = window->monitor->number;
     }