cursor-tracker: Add API to keep the wayland pointer focus while hidden

Since commit a2a8f0cda we force the focus surface of the
meta-wayland-pointer to NULL while the pointer is invisible. This
introduced an issue with the a11y magnifier of the shell, which uses
`meta_cursor_tracker_set_pointer_visible` to hide the real cursor and
show its own magnified cursor at the correct position: Because the
meta-wayland-pointer is still used to communicate with Wayland clients,
the UI of the windows will not respond to mouse movement anymore as
soon as the real cursor is hidden.

Fix this issue for now by adding an additional method to the
cursor-tracker which allows disabling the behavior commit a2a8f0cda
introduced.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/832

src/backends/meta-cursor-tracker-private.h

@@ -31,6 +31,7 @@ struct _MetaCursorTracker {
   GObject parent_instance;
 
   gboolean is_showing;
+  gboolean keep_focus_while_hidden;
 
   MetaCursorSprite *effective_cursor; /* May be NULL when hidden */
   MetaCursorSprite *displayed_cursor;

src/backends/meta-cursor-tracker.c

@@ -136,6 +136,7 @@ static void
 meta_cursor_tracker_init (MetaCursorTracker *self)
 {
   self->is_showing = TRUE;
+  self->keep_focus_while_hidden = FALSE;
 }
 
 static void
@@ -457,6 +458,44 @@ meta_cursor_tracker_set_pointer_visible (MetaCursorTracker *tracker,
   g_signal_emit (tracker, signals[VISIBILITY_CHANGED], 0);
 }
 
+/**
+ * meta_cursor_tracker_get_keep_focus_while_hidden:
+ * @tracker: a #MetaCursorTracker object.
+ *
+ * Returns: %FALSE if the Wayland focus surface of the pointer will
+ * be forced to NULL while the pointer is hidden, %TRUE otherwise.
+ * This function is only meant to be used by the magnifier of the shell
+ * and will be removed in a future release.
+ */
+gboolean
+meta_cursor_tracker_get_keep_focus_while_hidden (MetaCursorTracker *tracker)
+{
+  return tracker->keep_focus_while_hidden;
+}
+
+/**
+ * meta_cursor_tracker_set_keep_focus_while_hidden:
+ * @tracker: a #MetaCursorTracker object.
+ * @keep_focus: whether to keep the cursor focus while hidden
+ *
+ * If this is set to %TRUE, the Wayland focus surface of the pointer will
+ * not be forced to NULL while the pointer is hidden.
+ * This function is only meant to be used by the magnifier of the shell
+ * and will be removed in a future release.
+ */
+void
+meta_cursor_tracker_set_keep_focus_while_hidden (MetaCursorTracker *tracker,
+                                                 gboolean           keep_focus)
+{
+  if (keep_focus == tracker->keep_focus_while_hidden)
+    return;
+  tracker->keep_focus_while_hidden = keep_focus;
+
+  sync_cursor (tracker);
+
+  g_signal_emit (tracker, signals[VISIBILITY_CHANGED], 0);
+}
+
 MetaCursorSprite *
 meta_cursor_tracker_get_displayed_cursor (MetaCursorTracker *tracker)
 {

src/meta/meta-cursor-tracker.h

@@ -62,4 +62,11 @@ META_EXPORT
 void           meta_cursor_tracker_set_pointer_visible (MetaCursorTracker *tracker,
                                                         gboolean           visible);
 
+META_EXPORT
+gboolean       meta_cursor_tracker_get_keep_focus_while_hidden (MetaCursorTracker *tracker);
+
+META_EXPORT
+void           meta_cursor_tracker_set_keep_focus_while_hidden (MetaCursorTracker *tracker,
+                                                                gboolean           keep_focus);
+
 #endif

src/wayland/meta-wayland-pointer.c

@@ -228,7 +228,8 @@ sync_focus_surface (MetaWaylandPointer *pointer)
   MetaBackend *backend = meta_get_backend ();
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
 
-  if (!meta_cursor_tracker_get_pointer_visible (cursor_tracker))
+  if (!meta_cursor_tracker_get_pointer_visible (cursor_tracker) &&
+      !meta_cursor_tracker_get_keep_focus_while_hidden (cursor_tracker))
     {
       meta_wayland_pointer_set_focus (pointer, NULL);
       return;
@@ -433,7 +434,8 @@ default_grab_focus (MetaWaylandPointerGrab *grab,
   if (!meta_wayland_seat_has_pointer (seat))
     return;
 
-  if (!meta_cursor_tracker_get_pointer_visible (cursor_tracker))
+  if (!meta_cursor_tracker_get_pointer_visible (cursor_tracker) &&
+      !meta_cursor_tracker_get_keep_focus_while_hidden (cursor_tracker))
     return;
 
   if (pointer->button_count > 0)
@@ -898,7 +900,8 @@ meta_wayland_pointer_set_focus (MetaWaylandPointer *pointer,
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
 
   g_return_if_fail (meta_cursor_tracker_get_pointer_visible (cursor_tracker) ||
-		    surface == NULL);
+                    meta_cursor_tracker_get_keep_focus_while_hidden (cursor_tracker) ||
+                    surface == NULL);
 
   if (pointer->focus_surface == surface)
     return;