xwayland: add _XWAYLAND_MAY_GRAB_KEYBOARD property

Add a new client message "_XWAYLAND_MAY_GRAB_KEYBOARD" that X11 clients
can use to tell mutter this is a well behaving X11 client so it may
grant the keyboard grabs when requested.

An X11 client wishing to be granted Xwayland grabs by gnome-shell/mutter
must send a ClientMessage to the root window with:

 - message_type set to "_XWAYLAND_MAY_GRAB_KEYBOARD"
 - window set to the xid of the window on which the grab is to be issued
 - data.l[0] to a non-zero value

Note: Sending this client message when running a plain native X11
environment would have no effect.

https://bugzilla.gnome.org/show_bug.cgi?id=783342

src/wayland/meta-window-xwayland.c

@@ -23,9 +23,22 @@
 #include "wayland/meta-window-xwayland.h"
 #include "wayland/meta-wayland.h"
 
+enum
+{
+  PROP_0,
+
+  PROP_XWAYLAND_MAY_GRAB_KEYBOARD,
+
+  PROP_LAST
+};
+
+static GParamSpec *obj_props[PROP_LAST];
+
 struct _MetaWindowXwayland
 {
   MetaWindowX11 parent;
+
+  gboolean xwayland_may_grab_keyboard;
 };
 
 struct _MetaWindowXwaylandClass
@@ -58,11 +71,62 @@ meta_window_xwayland_shortcuts_inhibited (MetaWindow         *window,
   return meta_wayland_compositor_is_shortcuts_inhibited (compositor, source);
 }
 
+static void
+meta_window_xwayland_get_property (GObject    *object,
+                                   guint       prop_id,
+                                   GValue     *value,
+                                   GParamSpec *pspec)
+{
+  MetaWindowXwayland *window = META_WINDOW_XWAYLAND (object);
+
+  switch (prop_id)
+    {
+    case PROP_XWAYLAND_MAY_GRAB_KEYBOARD:
+      g_value_set_boolean (value, window->xwayland_may_grab_keyboard);
+      break;
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
+      break;
+    }
+}
+
+static void
+meta_window_xwayland_set_property (GObject      *object,
+                                   guint         prop_id,
+                                   const GValue *value,
+                                   GParamSpec   *pspec)
+{
+  MetaWindowXwayland *window = META_WINDOW_XWAYLAND (object);
+
+  switch (prop_id)
+    {
+    case PROP_XWAYLAND_MAY_GRAB_KEYBOARD:
+      window->xwayland_may_grab_keyboard = g_value_get_boolean (value);
+      break;
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
+      break;
+    }
+}
+
 static void
 meta_window_xwayland_class_init (MetaWindowXwaylandClass *klass)
 {
   MetaWindowClass *window_class = META_WINDOW_CLASS (klass);
+  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
 
   window_class->force_restore_shortcuts = meta_window_xwayland_force_restore_shortcuts;
   window_class->shortcuts_inhibited = meta_window_xwayland_shortcuts_inhibited;
+
+  gobject_class->get_property = meta_window_xwayland_get_property;
+  gobject_class->set_property = meta_window_xwayland_set_property;
+
+  obj_props[PROP_XWAYLAND_MAY_GRAB_KEYBOARD] =
+    g_param_spec_boolean ("xwayland-may-grab-keyboard",
+                          "Xwayland may use keyboard grabs",
+                          "Whether the client may use Xwayland keyboard grabs on this window",
+                          FALSE,
+                          G_PARAM_READWRITE);
+
+  g_object_class_install_properties (gobject_class, PROP_LAST, obj_props);
 }

src/wayland/meta-xwayland.h

@@ -37,4 +37,8 @@ meta_xwayland_handle_wl_surface_id (MetaWindow *window,
 gboolean
 meta_xwayland_is_xwayland_surface (MetaWaylandSurface *surface);
 
+void
+meta_xwayland_handle_xwayland_grab (MetaWindow *window,
+                                    gboolean    allow);
+
 #endif /* META_XWAYLAND_H */

src/x11/atomnames.h

@@ -81,6 +81,7 @@ item(VERSION)
 item(ATOM_PAIR)
 item(_XKB_RULES_NAMES)
 item(WL_SURFACE_ID)
+item(_XWAYLAND_MAY_GRAB_KEYBOARD)
 
 /* Oddities: These are used, and we need atoms for them,
  * but when we need all _NET_WM hints (i.e. when we're making

src/x11/events.c

@@ -1515,6 +1515,13 @@ handle_other_xevent (MetaDisplay *display,
               guint32 surface_id = event->xclient.data.l[0];
               meta_xwayland_handle_wl_surface_id (window, surface_id);
             }
+          else if (event->xclient.message_type == display->atom__XWAYLAND_MAY_GRAB_KEYBOARD)
+            {
+              if (meta_is_wayland_compositor ())
+                g_object_set (G_OBJECT (window),
+                              "xwayland-may-grab-keyboard", (event->xclient.data.l[0] != 0),
+                              NULL);
+            }
           else
 #endif
           if (!frame_was_receiver)