wlr_scene: Handle fractional scaling better

Try to alleviate scaling inaccuracies by implementing a fudge factor.

types/scene/wlr_scene.c

@@ -270,6 +270,14 @@ static uint32_t region_area(pixman_region32_t *region) {
 	return area;
 }
 
+static void scale_output_damage(pixman_region32_t *damage, float scale) {
+	wlr_region_scale(damage, damage, scale);
+
+	if (floor(scale) != scale) {
+		wlr_region_expand(damage, damage, 1);
+	}
+}
+
 static void scene_damage_outputs(struct wlr_scene *scene, pixman_region32_t *damage) {
 	if (!pixman_region32_not_empty(damage)) {
 		return;
@@ -282,8 +290,7 @@ static void scene_damage_outputs(struct wlr_scene *scene, pixman_region32_t *dam
 		pixman_region32_copy(&output_damage, damage);
 		pixman_region32_translate(&output_damage,
 			-scene_output->x, -scene_output->y);
-		wlr_region_scale(&output_damage, &output_damage,
+		scale_output_damage(&output_damage, scene_output->output->scale);
-			scene_output->output->scale);
 		if (wlr_damage_ring_add(&scene_output->damage_ring, &output_damage)) {
 			wlr_output_schedule_frame(scene_output->output);
 		}
@@ -649,6 +656,11 @@ void wlr_scene_buffer_set_buffer_with_damage(struct wlr_scene_buffer *scene_buff
 		pixman_region32_intersect(&output_damage, &output_damage, &cull_region);
 		pixman_region32_fini(&cull_region);
 
+		// if we are using fractional scaling add a 1px margin.
+		if (floor(output_scale) != output_scale) {
+			wlr_region_expand(&output_damage, &output_damage, 1);
+		}
+
 		pixman_region32_translate(&output_damage,
 			(lx - scene_output->x) * output_scale,
 			(ly - scene_output->y) * output_scale);
@@ -1055,7 +1067,7 @@ static void scene_node_render(struct wlr_scene_node *node,
 	pixman_region32_init(&render_region);
 	pixman_region32_copy(&render_region, &node->visible);
 	pixman_region32_translate(&render_region, -scene_output->x, -scene_output->y);
-	wlr_region_scale(&render_region, &render_region, output->scale);
+	scale_output_damage(&render_region, output->scale);
 	pixman_region32_intersect(&render_region, &render_region, damage);
 	if (!pixman_region32_not_empty(&render_region)) {
 		pixman_region32_fini(&render_region);
@@ -1534,6 +1546,8 @@ bool wlr_scene_output_commit(struct wlr_scene_output *scene_output) {
 	// scene nodes above. Those scene nodes will just render atop having us
 	// never see the background.
 	if (scene_output->scene->calculate_visibility) {
+		float output_scale = scene_output->output->scale;
+
 		for (int i = list_len - 1; i >= 0; i--) {
 			struct wlr_scene_node *node = list_data[i];
 			int x, y;
@@ -1550,10 +1564,18 @@ bool wlr_scene_output_commit(struct wlr_scene_output *scene_output) {
 			pixman_region32_intersect(&opaque, &opaque, &node->visible);
 
 			pixman_region32_translate(&opaque, -scene_output->x, -scene_output->y);
-			wlr_region_scale(&opaque, &opaque, scene_output->output->scale);
+			wlr_region_scale(&opaque, &opaque, output_scale);
 			pixman_region32_subtract(&background, &background, &opaque);
 			pixman_region32_fini(&opaque);
 		}
+
+		if (floor(output_scale) != output_scale) {
+			wlr_region_expand(&background, &background, 1);
+
+			// reintersect with the damage because we never want to render
+			// outside of the damage region
+			pixman_region32_intersect(&background, &background, &damage);
+		}
 	}
 
 	int nrects;