wlr_scene: Add primary_output property to wlr_scene_buffer

include/wlr/types/wlr_scene.h

@@ -117,6 +117,14 @@ struct wlr_scene_buffer {
 		struct wl_signal output_present; // struct wlr_scene_output
 	} events;
 
+	/**
+	 * The output that the largest area of this buffer is displayed on.
+	 * This may be NULL if the buffer is not currently displayed on any
+	 * outputs. This is the output that should be used for frame callbacks,
+	 * presentation feedback, etc.
+	 */
+	struct wlr_scene_output *primary_output;
+
 	// private state
 
 	uint64_t active_outputs;

types/scene/wlr_scene.c

@@ -218,6 +218,9 @@ static void scene_buffer_update_outputs(
 	struct wlr_box buffer_box = { .x = lx, .y = ly };
 	scene_node_get_size(&scene_buffer->node, &buffer_box.width, &buffer_box.height);
 
+	int largest_overlap = 0;
+	scene_buffer->primary_output = NULL;
+
 	struct wlr_scene_output *scene_output;
 	wl_list_for_each(scene_output, &scene->outputs, link) {
 		struct wlr_box output_box = {
@@ -231,6 +234,14 @@ static void scene_buffer_update_outputs(
 		bool intersects = wlr_box_intersection(&intersection, &buffer_box, &output_box);
 		bool intersects_before = scene_buffer->active_outputs & (1ull << scene_output->index);
 
+		if (intersects) {
+			int overlap = intersection.width * intersection.height;
+			if (overlap > largest_overlap) {
+				largest_overlap = overlap;
+				scene_buffer->primary_output = scene_output;
+			}
+		}
+
 		if (intersects && !intersects_before) {
 			scene_buffer->active_outputs |= 1ull << scene_output->index;
 			wlr_signal_emit_safe(&scene_buffer->events.output_enter, scene_output);