input: Propagate modifiers to clients with only pointer focus

When focus_follows_mouse is off, it is possible to hover and/or scroll
windows without giving them keyboard focus.  However, attempts to use
Ctrl+scroll on such clients will not work correctly because the keyboard
modifiers are not available.  This patch propagates modifier state to
the client with pointer focus when it gains focus and when the state
changes.

This depends on https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4523.

https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/259
discusses permitting this behavior at the protocol level.

sway/input/keyboard.c

@@ -659,10 +659,13 @@ static void handle_modifier_event(struct sway_keyboard *keyboard) {
 			wlr_input_method_keyboard_grab_v2_send_modifiers(kb_grab,
 					&keyboard->wlr->modifiers);
 		} else {
-			struct wlr_seat *wlr_seat = keyboard->seat_device->sway_seat->wlr_seat;
+			struct sway_seat *seat = keyboard->seat_device->sway_seat;
+			struct wlr_seat *wlr_seat = seat->wlr_seat;
 			wlr_seat_set_keyboard(wlr_seat, keyboard->wlr);
 			wlr_seat_keyboard_notify_modifiers(wlr_seat,
 					&keyboard->wlr->modifiers);
+			wlr_seat_client_notify_modifiers(wlr_seat,
+					wlr_seat->pointer_state.focused_client);
 		}
 
 		uint32_t modifiers = wlr_keyboard_get_modifiers(keyboard->wlr);

sway/input/seatop_default.c

@@ -609,15 +609,28 @@ static void handle_pointer_motion(struct sway_seat *seat, uint32_t time_msec) {
 	double sx, sy;
 	struct sway_node *node = node_at_coords(seat,
 			cursor->cursor->x, cursor->cursor->y, &surface, &sx, &sy);
+	struct wlr_seat_client *prev_focus_seat_client;
+	struct wlr_seat_client *curr_focus_seat_client;
+	struct wl_client *prev_focus_client;
+	struct wl_client *curr_focus_client;
 
 	if (config->focus_follows_mouse != FOLLOWS_NO) {
 		check_focus_follows_mouse(seat, e, node);
 	}
 
 	if (surface) {
 		if (seat_is_input_allowed(seat, surface)) {
+			prev_focus_seat_client = seat->wlr_seat->pointer_state.focused_client;
+			prev_focus_client = prev_focus_seat_client ? prev_focus_seat_client->client : NULL;
+
 			wlr_seat_pointer_notify_enter(seat->wlr_seat, surface, sx, sy);
 			wlr_seat_pointer_notify_motion(seat->wlr_seat, time_msec, sx, sy);
+
+			curr_focus_seat_client = seat->wlr_seat->pointer_state.focused_client;
+			curr_focus_client = curr_focus_seat_client ? curr_focus_seat_client->client : NULL;
+
+			if (prev_focus_client != curr_focus_client)
+				wlr_seat_client_notify_modifiers(seat->wlr_seat, curr_focus_seat_client);
 		}
 	} else {
 		cursor_update_image(cursor, node);