backend/drm: don't set cursor if off-screen

Closes: https://github.com/swaywm/wlroots/issues/2216

backend/drm/atomic.c

@@ -219,7 +219,7 @@ static bool atomic_crtc_commit(struct wlr_drm_backend *drm,
 		}
 		set_plane_props(&atom, drm, crtc->primary, crtc->id, 0, 0);
 		if (crtc->cursor) {
-			if (crtc->cursor->cursor_enabled) {
+			if (drm_connector_is_cursor_visible(conn)) {
 				set_plane_props(&atom, drm, crtc->cursor, crtc->id,
 					conn->cursor_x, conn->cursor_y);
 			} else {

backend/drm/drm.c

@@ -999,6 +999,17 @@ static bool drm_connector_move_cursor(struct wlr_output *output,
 	return true;
 }
 
+bool drm_connector_is_cursor_visible(struct wlr_drm_connector *conn) {
+	assert(conn->crtc != NULL && conn->crtc->cursor != NULL);
+	struct wlr_drm_plane *plane = conn->crtc->cursor;
+
+	return plane->cursor_enabled &&
+		conn->cursor_x < conn->output.width &&
+		conn->cursor_y < conn->output.height &&
+		conn->cursor_x + (int)plane->surf.width >= 0 &&
+		conn->cursor_y + (int)plane->surf.height >= 0;
+}
+
 static void drm_connector_destroy(struct wlr_output *output) {
 	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
 	drm_connector_cleanup(conn);

backend/drm/legacy.c

@@ -62,7 +62,7 @@ static bool legacy_crtc_commit(struct wlr_drm_backend *drm,
 		}
 	}
 
-	if (cursor != NULL && cursor->cursor_enabled) {
+	if (cursor != NULL && drm_connector_is_cursor_visible(conn)) {
 		struct wlr_drm_fb *cursor_fb = plane_get_next_fb(cursor);
 		struct gbm_bo *cursor_bo =
 			drm_fb_acquire(cursor_fb, drm, &cursor->mgpu_surf);

include/backend/drm/drm.h

@@ -152,6 +152,7 @@ void scan_drm_connectors(struct wlr_drm_backend *state);
 int handle_drm_event(int fd, uint32_t mask, void *data);
 bool drm_connector_set_mode(struct wlr_drm_connector *conn,
 	struct wlr_output_mode *mode);
+bool drm_connector_is_cursor_visible(struct wlr_drm_connector *conn);
 size_t drm_crtc_get_gamma_lut_size(struct wlr_drm_backend *drm,
 	struct wlr_drm_crtc *crtc);