fix flickering when resizing/spawning windows

Fixes: djpohly#306

client.h

@@ -197,6 +197,21 @@ client_is_mapped(Client *c)
 	return c->surface.xdg->mapped;
 }
 
+static inline int
+client_is_rendered_on_mon(Client *c, Monitor *m)
+{
+	/* This is needed for when you don't want to check formal assignment,
+	 * but rather actual displaying of the pixels.
+	 * Usually VISIBLEON suffices and is also faster. */
+	struct wlr_surface_output *s;
+	if (!c->scene->node.enabled)
+		return 0;
+	wl_list_for_each(s, &client_surface(c)->current_outputs, link)
+		if (s->output == m->wlr_output)
+			return 1;
+	return 0;
+}
+
 static inline int
 client_is_unmanaged(Client *c)
 {

dwl.c

@@ -182,7 +182,6 @@ struct Monitor {
 	unsigned int tagset[2];
 	double mfact;
 	int nmaster;
-	int un_map; /* If a map/unmap happened on this monitor, then this should be true */
 };
 
 typedef struct {
@@ -1393,8 +1392,6 @@ mapnotify(struct wl_listener *listener, void *data)
 	}
 	printstatus();
 
-	c->mon->un_map = 1;
-
 unset_fullscreen:
 	m = c->mon ? c->mon : xytomon(c->geom.x, c->geom.y);
 	wl_list_for_each(w, &clients, link)
@@ -1693,30 +1690,19 @@ rendermon(struct wl_listener *listener, void *data)
 	 * generally at the output's refresh rate (e.g. 60Hz). */
 	Monitor *m = wl_container_of(listener, m, frame);
 	Client *c;
-	int skip = 0;
 	struct timespec now;
 
-	clock_gettime(CLOCK_MONOTONIC, &now);
-
 	/* Render if no XDG clients have an outstanding resize and are visible on
 	 * this monitor. */
-	/* Checking m->un_map for every client is not optimal but works */
-	wl_list_for_each(c, &clients, link) {
-		if ((c->resize && m->un_map) || (c->type == XDGShell
-				&& (c->surface.xdg->pending.geometry.width !=
-				c->surface.xdg->current.geometry.width
-				|| c->surface.xdg->pending.geometry.height !=
-				c->surface.xdg->current.geometry.height))) {
-			/* Lie */
-			wlr_surface_send_frame_done(client_surface(c), &now);
-			skip = 1;
-		}
-	}
-	if (!skip && !wlr_scene_output_commit(m->scene_output))
+	wl_list_for_each(c, &clients, link)
+		if (client_is_rendered_on_mon(c, m) && (!c->isfloating && c->resize))
+			goto skip;
+	if (!wlr_scene_output_commit(m->scene_output))
 		return;
+skip:
 	/* Let clients know a frame has been rendered */
+	clock_gettime(CLOCK_MONOTONIC, &now);
 	wlr_scene_output_send_frame_done(m->scene_output, &now);
-	m->un_map = 0;
 }
 
 void
@@ -2281,9 +2267,6 @@ unmapnotify(struct wl_listener *listener, void *data)
 		grabc = NULL;
 	}
 
-	if (c->mon)
-		c->mon->un_map = 1;
-
 	if (client_is_unmanaged(c)) {
 		if (c == exclusive_focus)
 			exclusive_focus = NULL;