settings: Slack off “xwayland-allow-grabs” setting

To emulate X11 grabs, mutter as a Wayland compositor would disable its
own keyboard shortcuts and when the X11 window is an override redirect
window (which never receives focus), it also forces keyboard focus onto
that X11 O-R window so that all keyboard events are routed to the
window, just like an X11 server would.

But that's a bit of a “all-or-nothing” approach which prevents
applications that would legitimately grab the keyboard under X11 (like
virtual machine viewers) to work by default.

Change “xwayland-allow-grabs” to control whether the keyboard focus
should be locked onto override redirect windows in case of an X11 grab.

For stringent needs, careful users can still use the blacklisting
feature (i.e. a list containing “!*”) to prevent grabs from any X11
applications to affect other Wayland native applications.

https://gitlab.gnome.org/GNOME/mutter/issues/597

data/org.gnome.mutter.wayland.gschema.xml.in

@@ -61,10 +61,17 @@
 
     <key name="xwayland-allow-grabs" type="b">
       <default>false</default>
-      <summary>Allow grabs with Xwayland</summary>
+      <summary>Allow X11 grabs to lock keyboard focus with Xwayland</summary>
       <description>
-        Allow keyboard grabs issued by X11 applications running in Xwayland
+        Allow all keyboard events to be routed to X11 “override redirect”
-        to be taken into account.
+        windows with a grab when running in Xwayland.
+
+        This option is to support X11 clients which map an “override redirect”
+        window (which do not receive keyboard focus) and issue a keyboard
+        grab to force all keyboard events to that window.
+
+        This option is seldom used and has no effect on regular X11 windows
+        which can receive keyboard focus under normal circumstances.
 
         For a X11 grab to be taken into account under Wayland, the client must
         also either send a specific X11 ClientMessage to the root window or be

src/wayland/meta-xwayland-grab-keyboard.c

@@ -193,8 +193,6 @@ meta_xwayland_grab_is_granted (MetaWindow *window)
 
   backend = meta_get_backend ();
   settings = meta_backend_get_settings (backend);
-  if (!meta_settings_are_xwayland_grabs_allowed (settings))
-    return FALSE;
 
   /* Check whether the window is blacklisted */
   meta_settings_get_xwayland_grab_patterns (settings, &whitelist, &blacklist);
@@ -214,6 +212,22 @@ meta_xwayland_grab_is_granted (MetaWindow *window)
   return FALSE;
 }
 
+static gboolean
+meta_xwayland_grab_should_lock_focus (MetaWindow *window)
+{
+  MetaBackend *backend;
+  MetaSettings *settings;
+
+  /* Lock focus applies to O-R windows which never receive keyboard focus otherwise */
+  if (!window->override_redirect)
+    return FALSE;
+
+  backend = meta_get_backend ();
+  settings = meta_backend_get_settings (backend);
+
+  return meta_settings_are_xwayland_grabs_allowed (settings);
+}
+
 static void
 meta_xwayland_keyboard_grab_activate (MetaXwaylandKeyboardActiveGrab *active_grab)
 {
@@ -225,8 +239,8 @@ meta_xwayland_keyboard_grab_activate (MetaXwaylandKeyboardActiveGrab *active_gra
     {
       meta_verbose ("XWayland window %s has a grab granted", window->desc);
       meta_wayland_surface_inhibit_shortcuts (surface, seat);
-      /* Use a grab for O-R windows which never receive keyboard focus otherwise */
+
-      if (window->override_redirect)
+      if (meta_xwayland_grab_should_lock_focus (window))
         meta_wayland_keyboard_start_grab (seat->keyboard, &active_grab->keyboard_grab);
     }
   if (active_grab->window_associate_handler)