Rework session lock keyboard focus handling

When removing outputs, it is possible to end up in a situation where
none of the session lock client's surfaces have keyboard focus,
resulting in it not receiving keyboard events.  Track the focused
surface and update it as needed on surface destroy.

include/sway/input/seat.h

@@ -201,10 +201,6 @@ struct sway_workspace *seat_get_last_known_workspace(struct sway_seat *seat);
 
 struct sway_container *seat_get_focused_container(struct sway_seat *seat);
 
-// Force focus to a particular surface that is not part of the workspace
-// hierarchy (used for lockscreen)
-void sway_force_focus(struct wlr_surface *surface);
-
 /**
  * Return the last container to be focused for the seat (or the most recently
  * opened if no container has received focused) that is a child of the given

include/sway/server.h

@@ -96,6 +96,7 @@ struct sway_server {
 		struct wlr_session_lock_manager_v1 *manager;
 
 		struct wlr_session_lock_v1 *lock;
+		struct wlr_surface *focused;
 		struct wl_listener lock_new_surface;
 		struct wl_listener lock_unlock;
 		struct wl_listener lock_destroy;

sway/input/seat.c

@@ -214,15 +214,6 @@ static void seat_send_focus(struct sway_node *node, struct sway_seat *seat) {
 	}
 }
 
-void sway_force_focus(struct wlr_surface *surface) {
-	struct sway_seat *seat;
-	wl_list_for_each(seat, &server.input->seats, link) {
-		seat_keyboard_notify_enter(seat, surface);
-		seat_tablet_pads_notify_enter(seat, surface);
-		sway_input_method_relay_set_focus(&seat->im_relay, surface);
-	}
-}
-
 void seat_for_each_node(struct sway_seat *seat,
 		void (*f)(struct sway_node *node, void *data), void *data) {
 	struct sway_seat_node *current = NULL;

sway/lock.c

@@ -17,9 +17,20 @@ struct sway_session_lock_surface {
 	struct wl_listener output_commit;
 };
 
+static void set_lock_focused_surface(struct wlr_surface *focused) {
+	server.session_lock.focused = focused;
+
+	struct sway_seat *seat;
+	wl_list_for_each(seat, &server.input->seats, link) {
+		seat_set_focus_surface(seat, focused, false);
+	}
+}
+
 static void handle_surface_map(struct wl_listener *listener, void *data) {
 	struct sway_session_lock_surface *surf = wl_container_of(listener, surf, map);
-	sway_force_focus(surf->surface);
+	if (server.session_lock.focused == NULL) {
+		set_lock_focused_surface(surf->surface);
+	}
 	output_damage_whole(surf->output);
 }
 
@@ -48,6 +59,21 @@ static void handle_output_commit(struct wl_listener *listener, void *data) {
 
 static void handle_surface_destroy(struct wl_listener *listener, void *data) {
 	struct sway_session_lock_surface *surf = wl_container_of(listener, surf, destroy);
+
+	// Move the seat focus to another surface if one is available
+	if (server.session_lock.focused == surf->surface) {
+		struct wlr_surface *next_focus = NULL;
+
+		struct wlr_session_lock_surface_v1 *other;
+		wl_list_for_each(other, &server.session_lock.lock->surfaces, link) {
+			if (other != surf->lock_surface && other->mapped) {
+				next_focus = other->surface;
+				break;
+			}
+		}
+		set_lock_focused_surface(next_focus);
+	}
+
 	wl_list_remove(&surf->map.link);
 	wl_list_remove(&surf->destroy.link);
 	wl_list_remove(&surf->surface_commit.link);
@@ -88,6 +114,7 @@ static void handle_unlock(struct wl_listener *listener, void *data) {
 	sway_log(SWAY_DEBUG, "session unlocked");
 	server.session_lock.locked = false;
 	server.session_lock.lock = NULL;
+	server.session_lock.focused = NULL;
 
 	wl_list_remove(&server.session_lock.lock_new_surface.link);
 	wl_list_remove(&server.session_lock.lock_unlock.link);
@@ -115,6 +142,7 @@ static void handle_unlock(struct wl_listener *listener, void *data) {
 static void handle_abandon(struct wl_listener *listener, void *data) {
 	sway_log(SWAY_INFO, "session lock abandoned");
 	server.session_lock.lock = NULL;
+	server.session_lock.focused = NULL;
 
 	wl_list_remove(&server.session_lock.lock_new_surface.link);
 	wl_list_remove(&server.session_lock.lock_unlock.link);