Reimplement focus-previous-window with MRU focus timestamps

Makes it consistent with the MRU and improves the behavior:
- considers MRU's debounce
- fixes problematic cases where focus changes to non-Layout and back

src/input/mod.rs

@@ -779,7 +779,19 @@ impl State {
                 self.niri.queue_redraw_all();
             }
             Action::FocusWindowPrevious => {
-                if let Some(window) = self.niri.previously_focused_window.clone() {
+                let current = self.niri.layout.focus().map(|win| win.id());
+                if let Some(window) = self
+                    .niri
+                    .layout
+                    .windows()
+                    .map(|(_, win)| win)
+                    .filter(|win| Some(win.id()) != current)
+                    .max_by_key(|win| win.get_focus_timestamp())
+                    .map(|win| win.window.clone())
+                {
+                    // Commit current focus so repeated focus-window-previous works as expected.
+                    self.niri.mru_apply_keyboard_commit();
+
                     self.focus_window(&window);
                 }
             }

src/niri.rs

@@ -325,7 +325,6 @@ pub struct Niri {
     pub bind_repeat_timer: Option<RegistrationToken>,
     pub keyboard_focus: KeyboardFocus,
     pub layer_shell_on_demand_focus: Option<LayerSurface>,
-    pub previously_focused_window: Option<Window>,
     pub idle_inhibiting_surfaces: HashSet<WlSurface>,
     pub is_fdo_idle_inhibited: Arc<AtomicBool>,
     pub keyboard_shortcuts_inhibiting_surfaces: HashMap<WlSurface, KeyboardShortcutsInhibitor>,
@@ -1232,14 +1231,12 @@ impl State {
             );
 
             // Tell the windows their new focus state for window rule purposes.
-            let mut previous_focus = None;
             if let KeyboardFocus::Layout {
                 surface: Some(surface),
             } = &self.niri.keyboard_focus
             {
                 if let Some((mapped, _)) = self.niri.layout.find_window_and_output_mut(surface) {
                     mapped.set_is_focused(false);
-                    previous_focus = Some(mapped.window.clone());
                 }
             }
             if let KeyboardFocus::Layout {
@@ -1283,31 +1280,6 @@ impl State {
                 }
             }
 
-            // Update the previous focus but only when staying focused on the layout.
-            //
-            // Case 1: opening and closing exclusive-keyboard layer-shell (e.g. app launcher). This
-            // involves going from Layout to LayerShell, then from LayerShell to Layout. The
-            // previously focused window should stay unchanged.
-            //
-            //     Case 1.5: opening layer-shell, in the background switching layout focus, closing
-            //     layer-shell. With the current logic, this won't update the previously focused
-            //     window, which is incorrect. But this case should be rare.
-            //
-            // Case 2: switching to an empty workspace, then hitting FocusWindowPrevious. The focus
-            // should go to the window that was just focused. The keyboard focus goes from Layout
-            // (with Some surface) to Layout (with None surface), so we update the previously
-            // focused window.
-            //
-            // FIXME: Ideally this should happen inside Layout itself, then there wouldn't be any
-            // problems with layer-shell, etc. Or a similar problem now with the Overview where we
-            // don't update the previously focused window because the keyboard focus is on the
-            // Overview rather than on the Layout.
-            if matches!(self.niri.keyboard_focus, KeyboardFocus::Layout { .. })
-                && matches!(focus, KeyboardFocus::Layout { .. })
-            {
-                self.niri.previously_focused_window = previous_focus;
-            }
-
             if let Some(grab) = self.niri.popup_grab.as_mut() {
                 if grab.has_keyboard_grab && Some(&grab.root) != focus.surface() {
                     trace!(
@@ -2787,7 +2759,6 @@ impl Niri {
             seat,
             keyboard_focus: KeyboardFocus::Layout { surface: None },
             layer_shell_on_demand_focus: None,
-            previously_focused_window: None,
             idle_inhibiting_surfaces: HashSet::new(),
             is_fdo_idle_inhibited: Arc::new(AtomicBool::new(false)),
             keyboard_shortcuts_inhibiting_surfaces: HashMap::new(),