gdk: Keep track of the last slave device used

This last slave device (stored per master) is used to fill
in the missing slave device in synthesized crossing events
not directly caused by a device event (ie. due to configure
events or grabs)

gdk/gdkdisplayprivate.h

@@ -75,6 +75,7 @@ typedef struct
   gdouble toplevel_x, toplevel_y;
   guint32 state;
   guint32 button;
+  GdkDevice *last_slave;
 } GdkPointerWindowInfo;
 
 typedef struct

gdk/gdkwindow.c

@@ -9065,7 +9065,7 @@ do_synthesize_crossing_event (gpointer data)
               _gdk_synthesize_crossing_events (display,
                                                pointer_info->window_under_pointer,
                                                new_window_under_pointer,
-                                               device, NULL,
+                                               device, pointer_info->last_slave,
                                                GDK_CROSSING_NORMAL,
                                                pointer_info->toplevel_x,
                                                pointer_info->toplevel_y,
@@ -9591,6 +9591,17 @@ _gdk_windowing_got_event (GdkDisplay *display,
     {
       GdkInputMode mode;
 
+      pointer_info = _gdk_display_get_pointer_info (display, device);
+
+      if (pointer_info)
+        {
+          if (source_device != pointer_info->last_slave &&
+              gdk_device_get_device_type (source_device) == GDK_DEVICE_TYPE_SLAVE)
+            pointer_info->last_slave = source_device;
+          else
+            source_device = pointer_info->last_slave;
+        }
+
       g_object_get (device, "input-mode", &mode, NULL);
       _gdk_display_device_grab_update (display, device, source_device, serial);
 
@@ -9609,8 +9620,6 @@ _gdk_windowing_got_event (GdkDisplay *display,
   if (!event_window)
     return;
 
-  pointer_info = _gdk_display_get_pointer_info (display, device);
-
 #ifdef DEBUG_WINDOW_PRINTING
   if (event->type == GDK_KEY_PRESS &&
       (event->key.keyval == 0xa7 ||

gdk/x11/gdkdisplay-x11.c

@@ -1579,9 +1579,12 @@ device_grab_update_callback (GdkDisplay *display,
                              gpointer    data,
                              gulong      serial)
 {
+  GdkPointerWindowInfo *pointer_info;
   GdkDevice *device = data;
 
-  _gdk_display_device_grab_update (display, device, NULL, serial);
+  pointer_info = _gdk_display_get_pointer_info (display, device);
+  _gdk_display_device_grab_update (display, device,
+                                   pointer_info->last_slave, serial);
 }
 
 #define XSERVER_TIME_IS_LATER(time1, time2)                        \