core: track if root is damaged

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend.c

@@ -106,6 +106,7 @@ bool paint_all_new(session_t *ps, struct managed_win *const t) {
 	// part of the image we want to reuse
 	region_t reg_damage;
 	reg_damage = get_damage(ps, ps->o.monitor_repaint || !ps->o.use_damage);
+	ps->root_damaged = false;
 
 	if (!pixman_region32_not_empty(&reg_damage)) {
 		pixman_region32_fini(&reg_damage);

src/common.h

@@ -264,6 +264,8 @@ typedef struct session {
 	bool render_queued;
 	/// For tracking damage regions
 	struct damage_ring damage_ring;
+	/// Whether the root image has been changed since last render
+	bool root_damaged;
 	/// Whether all windows are currently redirected.
 	bool redirected;
 	/// Pre-generated alpha pictures.

src/picom.c

@@ -1205,6 +1205,7 @@ void root_damaged(session_t *ps) {
 				log_error("Failed to bind root back pixmap");
 			}
 		}
+		ps->root_damaged = true;
 	}
 
 	// Mark screen damaged