scene: only skip bottom-most black rects with fractional scaling

We were relying on the fact that we wouldn't paint anything on top
of the black background in the region of a black rect. However
when fractional scaling is used the repaint region might get
expanded to nearby pixels by scale_output_damage(). As a result
the neighbour scene nodes might leak into the skipped black rect's
region.

Avoid this by using this optimization for bottom-most black rects
only when fractional scaling is used.

References: https://github.com/swaywm/sway/issues/8233

types/scene/wlr_scene.c

@@ -1504,6 +1504,7 @@ struct render_list_constructor_data {
 	struct wl_array *render_list;
 	bool calculate_visibility;
 	bool highlight_transparent_region;
+	bool fractional_scale;
 };
 
 static bool construct_render_list_iterator(struct wlr_scene_node *node,
@@ -1514,10 +1515,12 @@ static bool construct_render_list_iterator(struct wlr_scene_node *node,
 		return false;
 	}
 
-	// while rendering, the background should always be black.
+	// While rendering, the background should always be black. If we see a
-	// If we see a black rect, we can ignore rendering everything under the rect
+	// black rect, we can ignore rendering everything under the rect, and
-	// and even the rect itself.
+	// unless fractional scale is used even the rect itself (to avoid running
-	if (node->type == WLR_SCENE_NODE_RECT && data->calculate_visibility) {
+	// into issues regarding damage region expansion).
+	if (node->type == WLR_SCENE_NODE_RECT && data->calculate_visibility &&
+			(!data->fractional_scale || data->render_list->size == 0)) {
 		struct wlr_scene_rect *rect = wlr_scene_rect_from_node(node);
 		float *black = (float[4]){ 0.f, 0.f, 0.f, 1.f };
 
@@ -1773,6 +1776,7 @@ bool wlr_scene_output_build_state(struct wlr_scene_output *scene_output,
 		.render_list = &scene_output->render_list,
 		.calculate_visibility = scene_output->scene->calculate_visibility,
 		.highlight_transparent_region = scene_output->scene->highlight_transparent_region,
+		.fractional_scale = floor(render_data.scale) != render_data.scale,
 	};
 
 	list_con.render_list->size = 0;