<!DOCTYPE html>

<script>
function createResizeObserver(FunctionConstructor = Function) {
    const resizeObserver = new parent.ResizeObserver(new FunctionConstructor(`
        if (this.isInitialEntry) {
            this.isInitialEntry = false;
            return;
        }

        this.callbackInvocations++;
    `));

    resizeObserver.isInitialEntry = true;
    resizeObserver.callbackInvocations = 0;
    resizeObserver.observe(parent.resizeObserverTestRoot);

    return resizeObserver;
}
</script>