fix: #4233 `bind:offsetHeight` doesn't always fire

src/runtime/internal/dom.ts

@@ -624,6 +624,10 @@ export function add_resize_listener(node: HTMLElement, fn: () => void) {
 		iframe.src = 'about:blank';
 		iframe.onload = () => {
 			unsubscribe = listen(iframe.contentWindow, 'resize', fn);
+
+			// make sure an initial resize event is fired _after_ the iframe is loaded (which is asynchronous)
+			// see https://github.com/sveltejs/svelte/issues/4233
+			fn();
 		};
 	}