x11: Simplify handling of focus-follows-mouse

Focus follows mouse is meant to avoid focusing windows that happened
to pop up under the pointer, e.g. due to mapping, workspace changes,
etc... On X11, this has been done since ancient times through a
moderately complex synchronization mechanism, so mutter would know
to ignore crossing events caused on those situations.

This mechanism is much prior to XInput 2 though, where we may know
this in a more straightforward way: If the sourceid of the crossing
event is a logical pointer (i.e. equals deviceid), the crossing event
was triggered logically, and not through user input.

Perform this simpler check, and drop the existing mechanism to
ignore logically induced crossing events.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3267>

src/core/window.c

@@ -2203,7 +2203,6 @@ meta_window_show (MetaWindow *window)
   gboolean needs_stacking_adjustment;
   MetaWindow *focus_window;
   gboolean notify_demands_attention = FALSE;
-  MetaDisplay *display = window->display;
 
   meta_topic (META_DEBUG_WINDOW_STATE,
               "Showing window %s, iconic: %d placed: %d",
@@ -2373,16 +2372,6 @@ meta_window_show (MetaWindow *window)
           else
             meta_display_queue_focus (window->display, window);
         }
-      else if (display->x11_display)
-        {
-          /* Prevent EnterNotify events in sloppy/mouse focus from
-           * erroneously focusing the window that had been denied
-           * focus.  FIXME: This introduces a race; I have a couple
-           * ideas for a better way to accomplish the same thing, but
-           * they're more involved so do it this way for now.
-           */
-          meta_x11_display_increment_focus_sentinel (display->x11_display);
-        }
     }
 
   set_net_wm_state (window);

src/x11/atomnames.h

@@ -69,7 +69,6 @@ item(_GNOME_PANEL_ACTION)
 item(_GNOME_PANEL_ACTION_RUN_DIALOG)
 item(_MUTTER_TIMESTAMP_PING)
 item(_MUTTER_FOCUS_SET)
-item(_MUTTER_SENTINEL)
 item(_MUTTER_VERSION)
 item(_MUTTER_FRAME_FOR)
 item(_MUTTER_FRAME_EXTENTS)

src/x11/events.c

@@ -996,7 +996,7 @@ handle_input_xevent (MetaX11Display *x11_display,
           enter_event->mode != XINotifyUngrab &&
           enter_event->detail != XINotifyInferior &&
           !meta_is_wayland_compositor () &&
-          meta_x11_display_focus_sentinel_clear (x11_display))
+          enter_event->sourceid != enter_event->deviceid)
         {
           meta_window_handle_enter (window,
                                     enter_event->time,
@@ -1640,16 +1640,6 @@ handle_other_xevent (MetaX11Display *x11_display,
             else if (event->xproperty.atom ==
                      x11_display->atom__NET_DESKTOP_NAMES)
               meta_x11_display_update_workspace_names (x11_display);
-
-            /* we just use this property as a sentinel to avoid
-             * certain race conditions.  See the comment for the
-             * sentinel_counter variable declaration in display.h
-             */
-            if (event->xproperty.atom ==
-                x11_display->atom__MUTTER_SENTINEL)
-              {
-                meta_x11_display_decrement_focus_sentinel (x11_display);
-              }
           }
       }
       break;

src/x11/meta-x11-display-private.h

@@ -171,11 +171,6 @@ struct _MetaX11Display
    */
   unsigned long ignored_crossing_serials[N_IGNORED_CROSSING_SERIALS];
 
-  /* we use property updates as sentinels for certain window focus events
-   * to avoid some race conditions on EnterNotify events
-   */
-  int sentinel_counter;
-
   int composite_event_base;
   int composite_error_base;
   int composite_major_version;
@@ -265,10 +260,6 @@ MetaLogicalMonitor *meta_x11_display_xinerama_index_to_logical_monitor (MetaX11D
 void meta_x11_display_update_workspace_layout (MetaX11Display *x11_display);
 void meta_x11_display_update_workspace_names  (MetaX11Display *x11_display);
 
-void meta_x11_display_increment_focus_sentinel (MetaX11Display *x11_display);
-void meta_x11_display_decrement_focus_sentinel (MetaX11Display *x11_display);
-gboolean meta_x11_display_focus_sentinel_clear (MetaX11Display *x11_display);
-
 void meta_x11_display_update_focus_window (MetaX11Display *x11_display,
                                            Window          xwindow,
                                            gulong          serial,

src/x11/meta-x11-display.c

@@ -1091,25 +1091,6 @@ open_x_display (MetaDisplay  *display,
   return xdisplay;
 }
 
-static void
-on_window_visibility_updated (MetaDisplay    *display,
-                              GList          *placed_windows,
-                              GList          *shown_windows,
-                              GList          *hidden_windows,
-                              MetaX11Display *x11_display)
-{
-  GList *l;
-
-  if (meta_prefs_get_focus_mode () == G_DESKTOP_FOCUS_MODE_CLICK)
-    return;
-
-  if (display->mouse_mode)
-    return;
-
-  for (l = shown_windows; l; l = l->next)
-    meta_x11_display_increment_focus_sentinel (x11_display);
-}
-
 static void
 on_frames_client_died (GObject      *source,
                        GAsyncResult *result,
@@ -1285,10 +1266,6 @@ meta_x11_display_new (MetaDisplay  *display,
                            G_CALLBACK (update_cursor_theme),
                            x11_display,
                            G_CONNECT_SWAPPED);
-  g_signal_connect_object (display,
-                           "window-visibility-updated",
-                           G_CALLBACK (on_window_visibility_updated),
-                           x11_display, 0);
 
   g_signal_connect_object (display,
                            "x11-display-opened",
@@ -2356,38 +2333,6 @@ prefs_changed_callback (MetaPreference pref,
     }
 }
 
-void
-meta_x11_display_increment_focus_sentinel (MetaX11Display *x11_display)
-{
-  unsigned long data[1];
-
-  data[0] = meta_display_get_current_time (x11_display->display);
-
-  XChangeProperty (x11_display->xdisplay,
-                   x11_display->xroot,
-                   x11_display->atom__MUTTER_SENTINEL,
-                   XA_CARDINAL,
-                   32, PropModeReplace, (guchar*) data, 1);
-
-  x11_display->sentinel_counter += 1;
-}
-
-void
-meta_x11_display_decrement_focus_sentinel (MetaX11Display *x11_display)
-{
-  x11_display->sentinel_counter -= 1;
-
-  if (x11_display->sentinel_counter < 0)
-    x11_display->sentinel_counter = 0;
-}
-
-gboolean
-meta_x11_display_focus_sentinel_clear (MetaX11Display *x11_display)
-{
-  return (x11_display->sentinel_counter == 0);
-}
-
-
 static void
 meta_x11_display_add_ignored_crossing_serial (MetaX11Display *x11_display,
                                               unsigned long   serial)