It’s worth noting that this is one of those big differences between UXP and browsers – UXP’s DOM is not really synchronous. It looks like that most of the time, but layout is one of those places where it doesn’t.
UXP doesn’t calculate element sizes until a layout pass occurs, and those passes occur only when frames are generated. So if you add an element to the DOM, it could be upwards of 16ms before you see size information.
UXP tries to be helpful here: it will send a resize event your way when it’s safe to look at an element’s size, but of course you can use setTimeout or even requestAnimationFrame, but resize is the guaranteed method that works.
Of course, libraries made for the browser don’t do this, but they would have to be tweaked using timeouts or resize to work properly in UXP.