Is DOM rendering GUARANTEED to block during a single (synchronous) function's execution?
According to various sources, getting the position or size of a DOM element will trigger a reflow of the output if necessary, so that the returned values are correct. As a matter of fact, reading the offsetHeight of an element has become a way to force a reflow, as reported by Alexander Skutin and Daniel Norton.
Paul Irish gives a list of several actions that cause a reflow. Among them are these element box metrics methods and properties:
- elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight,
- elem.offsetParent elem.clientLeft, elem.clientTop, elem.clientWidth,
- elem.clientHeight elem.getClientRects(), elem.getBoundingClientRect()
Stoyan Stefanov describes strategies used by browsers to optimize reflows (e.g. queueing DOM changes and performing them in batches), and adds the following remark:
But sometimes the script may prevent the browser from optimizing the reflows, and cause it to flush the queue and perform all batched changes. This happens when you request style information, such as
- offsetTop, offsetLeft, offsetWidth, offsetHeight
- scrollTop/Left/Width/Height
- clientTop/Left/Width/Height
- getComputedStyle(), or currentStyle in IE
All of these above are essentially requesting style information about a node, and any time you do it, the browser has to give you the most up-to-date value. In order to do so, it needs to apply all scheduled changes, flush the queue, bite the bullet and do the reflow.
There is nothing in Javascript related to concurrency that is anything but de facto. JS simply does not define a concurrency model. Everything is happy accident or years of consensus.
That said, if your function does not make any calls to weird things like XMLHttpRequest or "alert" or something like that, you can basically treat it as single-threaded with no interrupts.