[gnome-mahjongg/arnaudb/wip/gtk4: 7/23] Make on_window_state_event work.

[Arnaud B. <arnaudb src gnome org>]

commit 4e783c07258804b3dfd91c27eb00028ae98f6d97
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Fri Mar 27 18:54:53 2020 +0100

    Make on_window_state_event work.

 src/gnome-mahjongg.vala | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)
---
diff --git a/src/gnome-mahjongg.vala b/src/gnome-mahjongg.vala
index b604e1a..d0cb733 100644
--- a/src/gnome-mahjongg.vala
+++ b/src/gnome-mahjongg.vala
@@ -79,7 +79,7 @@ public class Mahjongg : Gtk.Application
 
         window = new Gtk.ApplicationWindow (this);
         window.size_allocate.connect (size_allocate_cb);
-        window.window_state_event.connect (window_state_event_cb);
+        window.map.connect (init_state_watcher);
         window.set_default_size (settings.get_int ("window-width"), settings.get_int ("window-height"));
         if (settings.get_boolean ("window-is-maximized"))
             window.maximize ();
@@ -189,14 +189,27 @@ public class Mahjongg : Gtk.Application
         window.get_size (out window_width, out window_height);
     }
 
-    private bool window_state_event_cb (Gdk.EventWindowState event)
+    private void init_state_watcher ()
     {
-        if ((event.changed_mask & Gdk.WindowState.MAXIMIZED) != 0)
-            is_maximized = (event.new_window_state & Gdk.WindowState.MAXIMIZED) != 0;
-        /* We don’t save this state, but track it for saving size allocation */
-        if ((event.changed_mask & Gdk.WindowState.TILED) != 0)
-            is_tiled = (event.new_window_state & Gdk.WindowState.TILED) != 0;
-        return false;
+        Gdk.Surface? nullable_surface = window.get_surface ();      // TODO report bug, get_surface() 
returns a nullable Surface
+        if (nullable_surface == null || !((!) nullable_surface is Gdk.Toplevel))
+            assert_not_reached ();
+        surface = (Gdk.Toplevel) (!) nullable_surface;
+        surface.notify ["state"].connect (on_window_state_event);
+    }
+
+    private Gdk.Toplevel surface;
+    private const Gdk.SurfaceState tiled_state = Gdk.SurfaceState.TILED
+                                               | Gdk.SurfaceState.TOP_TILED
+                                               | Gdk.SurfaceState.BOTTOM_TILED
+                                               | Gdk.SurfaceState.LEFT_TILED
+                                               | Gdk.SurfaceState.RIGHT_TILED;
+    private void on_window_state_event ()
+    {
+        Gdk.SurfaceState state = surface.get_state ();
+
+        is_maximized =  (state & Gdk.SurfaceState.MAXIMIZED) != 0;
+        is_tiled =      (state & Gdk.SurfaceState.TILED)     != 0;
     }
 
     protected override void shutdown ()