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POIs DOM rendering GUARANTEED to block during a single (synchronous) function's execution?
 

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    <p>DOM blocking is something many people not familiar with JavaScript's strictly single-threaded synchronous execution model find out about the hard way, and it's usually just something we want to work around somehow (using timeouts, web-workers, etc). All well and good.</p> <p>However, I would like to know if blocking of the actual user-visible rendering is something you can actually <em>rely</em> on. I'm 90% sure it is <em>de facto</em> the case in most browsers but I am hoping this isn't just a happily consistent accident. I can't seem to find any definitive statements from DOM specifications or even vendor documentation like MDM.</p> <p>What worries me slightly is that while changes to the DOM are indeed not visible looking at the page, the internal DOM geometry (including CSS transforms and filters) <em>does</em> actually update during synchronous execution. For example:</p> <pre><code>console.log(element.getBoundingRect().width); element.classList.add("scale-and-rotate"); console.log(element.getBoundingRect().width); element.classList.remove("scale-and-rotate"); </code></pre> <p>... will indeed report two different width values, though the page does not <em>appear</em> to flash. Synchronously waiting after the class is added (using a while loop) doesn't make the temporary changes visible either. Doing a Timeline trace in Chrome reveals that internally paint and re-paint is taking place just the same, which makes sense...</p> <p>My concern is that, lacking a specific reason not, some browsers, like say, those dealing with underpowered mobile CPUs, may choose to actually reflect those internal calculations in the user-visible layout <em>during</em> that function's execution, and thus will result in an ugly "flash" during such temporary operations. So, more concretely, what I'm asking is: Do they have a specific reason not to?</p> <p>(If you are wondering why I care about this at all, I sometimes need to measure calculated dimensions using <code>getBoundingRect</code> for elements in a certain state to plan out spacing or animations or other such things, without actually putting them in that state or animating them first...)</p>
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    1. COUpdated values are updated to DOM instantly, but they are not _rendered_ before the script is finished, except: Looks like [window.scrollBy()](http://jsfiddle.net/6HXsP/) is updated during the execution in IEs. In the fiddle resize the "Result" window so small that you can scroll it, then click "Scroll" button.
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    2. CO@user2864740 how's that?
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    3. CO@Teemu - so what's your source for knowing that? I believe you of course, but I am specifically asking if this behaviour is something that *happens* to be true or if it's officially specced out somewhere and can be relied on. It certainly is worrying that there are exceptions :/
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