All of the below properties or methods, when requested/called in JavaScript, will force the browser to synchronously calculate the style and layout. This is also called reflow or layout thrashing, and is common performance bottleneck.

• elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight, elem.offsetParent
• elem.clientLeft, elem.clientTop, elem.clientWidth, elem.clientHeight
• elem.getClientRects(), elem.getBoundingClientRect()

• elem.scrollWidth, elem.scrollHeight
• elem.scrollLeft, elem.scrollTop // also setting them
• elem.scrollIntoView(), elem.scrollIntoViewIfNeeded()
• elem.scrollBy(), elem.scrollTo()

• elem.focus() // can trigger a *double* forced layout (source)

• elem.computedRole, elem.computedName
• elem.innerText (source)

• range.getClientRects(), range.getBoundingClientRect()

• mouseEvt.layerX, mouseEvt.layerY, mouseEvt.offsetX, mouseEvt.offsetY

source

window.getComputedStyle() // will typically force style recalc (source)

window.getComputedStyle() // will force layout, as well, if one of the following:

1. The element is in a shadow tree
2. There are media queries (viewport-related ones) specifically, one of the following: (source)

• min-width, min-height, max-width, max-height, width, height
• aspect-ratio, min-aspect-ratio, max-aspect-ratio
• device-pixel-ratio, resolution, orientation

1. The property requested is one of the following: (source)

• height, width
• top, right, bottom, left
• margin [-top, -right, -bottom, -left, or shorthand] only if the margin is fixed.
• padding [-top, -right, -bottom, -left, or shorthand] only if the padding is fixed.
• transform, transform-origin, perspective-origin
• translate, rotate, scale
• webkit-filter, backdrop-filter
• motion-path, motion-offset, motion-rotation
• x, y, rx, ry

• doc.scrollingElement // only forces style

• window.scrollX, window.scrollY

• window.innerHeight, window.innerWidth

• window.getMatchedCSSRules() // only forces style

• inputElem.select(), textareaElem.select()

(source)

• Lots & lots of stuff
• …including copying an image to clipboard (source)

• forced layout (and style recalc): updateLayoutIgnorePendingStylesheets - Chromium Code Search
• forced style recalc: updateLayoutTree - Chromium Code Search`