Oh boy - JavaScript and Flash really ARE on the same thread

Yeah, I know… this is something trivial that everyone knows, right? but I had to check it out.
The question I was presented with was: If I call a JavaScript function from Flash, will Flash UI keep on rendering or wait for a "response" from JS to continue?
The answer, as sad as it may be, is YES, it will stop rendering.

Here, check out the code below. It's a little test I did, where I have a button to launch an Alert from JS. I've also added a progress bar with it's indeterminate property set to "true" in order to see clearly if it stops, and there I have it. Once the Alert is up, the progress bar stops from rendering.
Hmmm… I don't like this. I don't like this at all.

Here is the Flex Application code:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application
xmlns:mx="http://www.adobe.com/2006/mxml"
layout="vertical">

<mx:Script>
<![CDATA[
private function onClick(event:MouseEvent):void {
if (ExternalInterface.available) {
ExternalInterface.call("showAlert");
}
}
]]>
</mx:Script>

<mx:Button
label="Show JS Alert"
click="onClick(event)"/>

<mx:ProgressBar indeterminate="true"/>
</mx:Application>

And here is what I've added to the HTML template:

function showAlert() {
alert("Flash UI stops from rendering!");
}