remove empty workspace when focusing its output

Until now, focus changing code only considered cleaning up the last focused
workspace. This commit adds removal of empty workspace from output that just
received focus on a different workspace.

Fixes: #1797

sway/input/seat.c

@@ -448,6 +448,21 @@ void seat_set_focus_warp(struct sway_seat *seat,
 		return;
 	}
 
+	struct sway_container *last_output = last_focus;
+	if (last_output && last_output->type != C_OUTPUT) {
+		last_output = container_parent(last_output, C_OUTPUT);
+	}
+	struct sway_container *new_output = container;
+	if (new_output && new_output->type != C_OUTPUT) {
+		new_output = container_parent(new_output, C_OUTPUT);
+	}
+
+	// find new output's old workspace, which might have to be removed if empty
+	struct sway_container *new_output_last_ws = NULL;
+	if (last_output && new_output && last_output != new_output) {
+		new_output_last_ws = seat_get_focus_by_type(seat, new_output, C_WORKSPACE);
+	}
+
 	if (container) {
 		struct sway_seat_container *seat_con =
 			seat_container_from_container(seat, container);
@@ -482,6 +497,14 @@ void seat_set_focus_warp(struct sway_seat *seat,
 		}
 	}
 
+	// clean up unfocused empty workspace on new output
+	if (new_output_last_ws) {
+		if (!workspace_is_visible(new_output_last_ws)
+			&& new_output_last_ws->children->length == 0) {
+			container_destroy(new_output_last_ws);
+		}
+	}
+
 	if (last_focus) {
 		struct sway_container *last_ws = last_focus;
 		if (last_ws && last_ws->type != C_WORKSPACE) {
@@ -499,14 +522,6 @@ void seat_set_focus_warp(struct sway_seat *seat,
 		}
 
 		if (config->mouse_warping && warp) {
-			struct sway_container *last_output = last_focus;
-			if (last_output && last_output->type != C_OUTPUT) {
-				last_output = container_parent(last_output, C_OUTPUT);
-			}
-			struct sway_container *new_output = container;
-			if (new_output && new_output->type != C_OUTPUT) {
-				new_output = container_parent(new_output, C_OUTPUT);
-			}
 			if (new_output && last_output && new_output != last_output) {
 				double x = new_output->x + container->x +
 					container->width / 2.0;