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.

Generally, all APIs that synchronously provide layout metrics will trigger forced reflow / layout. Read on for additional cases and details.

Element APIs

Getting box metrics

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

Scroll stuff

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

Setting focus

elem.focus() (source)

Also…

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

Getting window dimensions

window.scrollX, window.scrollY
window.innerHeight, window.innerWidth
window.visualViewport.height / width / offsetTop / offsetLeft (source)

document

document.scrollingElement only forces style
document.elementFromPoint

Forms: Setting selection + focus

inputElem.focus()
inputElem.select(), textareaElem.select()

Mouse events: Reading offset data

mouseEvt.layerX, mouseEvt.layerY, mouseEvt.offsetX, mouseEvt.offsetY (source)

Calling getComputedStyle()

window.getComputedStyle() will typically force style recalc.

window.getComputedStyle() will often force layout, as well.

Details of the conditions where gCS() forces layout

window.getComputedStyle() will force layout in one of 3 conditions:

The element is in a shadow tree
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 , min-device-pixel-ratio, max-device-pixel-ratio
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
- grid, grid-template, grid-template-columns, grid-template-rows
- perspective-origin
- These items were previously in the list but appear to not be any longer (as of Feb 2018): motion-path, motion-offset, motion-rotation, x, y, rx, ry

Getting `Range` dimensions

range.getClientRects(), range.getBoundingClientRect()

SVG

Quite a lot of properties/methods force, but I haven't made an exhaustive list. This list in incomplete:

SVGLocatable: computeCTM(), getBBox()
SVGTextContent: getCharNumAtPosition(), getComputedTextLength(), getEndPositionOfChar(), getExtentOfChar(), getNumberOfChars(), getRotationOfChar(), getStartPositionOfChar(), getSubStringLength(), selectSubString()
SVGUse: instanceRoot

Use the "chromium source tree link" below to explore on your own!

contenteditable

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

* Appendix

Reflow only has a cost if the document has changed and invalidated the style or layout. Typically, this is because the DOM was changed (classes modified, nodes added/removed, even adding a psuedo-class like :focus).
If layout is forced, style must be recalculated first. So forced layout triggers both operations. Their costs are very dependent on the content/situation, but typically both operations are similar in cost.
What should you do about all this? Well, the More on forced layout section below covers everything in more detail, but the short version is:
1. for loops that force layout & change the DOM are the worst, avoid them.
2. Use DevTools Performance Panel to see where this happens. You may be surprised to see how often your app code and library code hits this.
3. Batch your writes & reads to the DOM (via FastDOM or a virtual DOM implementation). Read your metrics at the begininng of the frame (very very start of rAF, scroll handler, etc), when the numbers are still identical to the last time layout was done.

Timeline trace of The Guardian. Outbrain is forcing layout repeatedly, probably in a loop.

Cross-browser

The above data was built by reading the Blink source, so it's true for Chrome, Opera, Brave, Edge and most android browsers. You can browse them yourself in the Chromium source tree.
Tony Gentilcore's Layout Triggering List was for 2011 WebKit and generally aligns with the above.
Modern WebKit's instances of forced layout are mostly consistent: updateLayoutIgnorePendingStylesheets - GitHub search - WebKit/WebKit
Gecko's reflow appears to be requested via FrameNeedsReflow. Results: FrameNeedsReflow - mozilla-central searchfox
No concrete data on IE or EdgeHTML, but they likely were roughly the same, as the return values for these properties are spec'd.

NikitaIT/what-forces-layout.md