What forces layout / reflow
All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout. This is also called reflow or layout thrashing, and is common performance bottleneck.
Element
Box metrics
elem.offsetLeft
,elem.offsetTop
,elem.offsetWidth
,elem.offsetHeight
,elem.offsetParent
elem.clientLeft
,elem.clientTop
,elem.clientWidth
,elem.clientHeight
elem.getClientRects()
,elem.getBoundingClientRect()