Issue #11957: also ignore events when switching to MWM.

This is a follow-up of commit c5db158f58, where the flickering issue was
happening when creating new windows when already in multi-window mode. I
had the same issue again when I had several tabs opened in SWM and
switched to MWM. Then it would undraw displays to move them to their own
window. In the short time when this occurs, we don't want to process
events. So now I changed the "drawn" status when undrawing ("unrealize"
event) a shell.

I also changed the implementation of gimp_displays_accept_focus_events().
What we need is to check the active shell only of each window. In SWM in
particular, it is normal that non-visible tab shells get unrealized.

app/display/gimpdisplay-foreach.c

@@ -32,6 +32,7 @@
 #include "gimpdisplay-foreach.h"
 #include "gimpdisplayshell.h"
 #include "gimpdisplayshell-cursor.h"
+#include "gimpimagewindow.h"
 
 
 gboolean
@@ -314,31 +315,34 @@ gimp_displays_unset_busy (Gimp *gimp)
  * When this returns %FALSE, we should ignore focus events. In
  * particular, I had weird cases in multi-window mode, before a new
  * image window was actually displayed on screen, there seemed to be
- * infinite loops of focus events switching the active display between
+ * infinite loops of focus events switching the active window between
  * the current active one and the one being created, with flickering
  * window title and the new display never actually displayed until we
  * broke the loop by manually getting out of focus (see #11957).
  * This is why when any of the display is not fully drawn yet (which
- * usually means it is being created), we temporarily ignore focus
- * events on all displays.
+ * may mean it is either being created or moved to a new window), we
+ * temporarily ignore focus events on all displays.
  */
 gboolean
 gimp_displays_accept_focus_events (Gimp *gimp)
 {
-  GList *list;
+  GList    *windows;
+  gboolean  accept = TRUE;
 
   g_return_val_if_fail (GIMP_IS_GIMP (gimp), FALSE);
 
-  for (list = gimp_get_display_iter (gimp);
-       list;
-       list = g_list_next (list))
+  windows = gimp_get_image_windows (gimp);
+  for (GList *iter = windows; iter; iter = iter->next)
     {
-      GimpDisplay      *display = list->data;
-      GimpDisplayShell *shell   = gimp_display_get_shell (display);
+      GimpDisplayShell *shell = gimp_image_window_get_active_shell (iter->data);
 
       if (! gimp_display_shell_is_drawn (shell))
-        return FALSE;
+        {
+          accept = FALSE;
+          break;
+        }
     }
+  g_list_free (windows);
 
-  return TRUE;
+  return accept;
 }

app/display/gimpdisplayshell.c

@@ -1045,6 +1045,11 @@ gimp_display_shell_unrealize (GtkWidget *widget)
     gtk_widget_unrealize (shell->nav_popup);
 
   GTK_WIDGET_CLASS (parent_class)->unrealize (widget);
+
+  shell->drawn = FALSE;
+  g_signal_connect (shell, "draw",
+                    G_CALLBACK (gimp_display_shell_draw),
+                    NULL);
 }
 
 static void