Keep track of flushing state

src/runtime/internal/scheduler.ts

@@ -31,8 +31,11 @@ export function add_flush_callback(fn) {
 	flush_callbacks.push(fn);
 }
 
+let flushing = false;
 const seen_callbacks = new Set();
 export function flush() {
+	if (flushing) return;
+	flushing = true;
 
 	do {
 		// first, call beforeUpdate functions
@@ -69,6 +72,7 @@ export function flush() {
 	}
 
 	update_scheduled = false;
+	flushing = false;
 	seen_callbacks.clear();
 }