What forces layout/reflow. The comprehensive list.

what-forces-layout.md

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:

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 , min-device-pixel-ratio, max-device-pixel-ratio
3. 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.

More on forced layout

• Avoiding layout thrashing — Web Fundamentals The best resource on identifying and fixing this topic.
• CSS Triggers - covers what operations are required as a result of setting/changing a given CSS value. The above list, however, are all about what forces the purple/green/darkgreen circles synchronously from JavaScript.
• Fixing Layout thrashing in the real world | Matt Andrews
• Timeline demo: Diagnosing forced synchronous layouts - Google Chrome
• Preventing 'layout thrashing' | Wilson Page
• wilsonpage/fastdom
• Rendering: repaint, reflow/relayout, restyle / Stoyan
• We spent a week making Trello boards load extremely fast. Here’s how we did it. - Fog Creek Blog
• Minimizing browser reflow  |  PageSpeed Insights  |  Google Developers
• Optimizing Web Content in UIWebViews and Websites on iOS
• Accelerated Rendering in Chrome
• web performance for the curious
• Jank Free

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Updated slightly April 2020. Codesearch links and a few changes to relevant element properties.

I tested scrollY inside window scroll event on Chrome Version 88.0.4324.150 and it doesn't fire Layout calculation.

The page is very simple, has 2000 words, so that there's a scrollbar to test.
I ran two tests by using the Performance Tab. One that has window scroll event that logs scrollY and the other that doesn't have js files.

Here are the comparisons of Tasks.

The one that doesn't have js files.

Scroll > Event:scroll > Update Layer Tree > Composite Layers 

The one that does log scrollY on the scroll event.

Scroll > Event:scroll > Update Layer Tree > Composite Layers 
         Function call
         window.addEventListener.passive

The one that logs scrollY is almost identical, except for the js part where you see Function call. If there was Layout thrashing, inside the Task there would be 'Layout'.

Here's a demo https://8mxws.csb.app/
Here's the source https://codesandbox.io/s/testscrollyreflow-8mxws?file=/app.js

I didn't test within codesandbox, I tested locally, but i'm sharing it that way cuz it's the easiest way

If every item in the scrollable list has a &:hover {... will this cause layout thrashing when scrolling?

Scroll > Event:scroll > Update Layer Tree > Composite Layers
@aquaductape how do I get same output for debugging? any way to see what stages getting executed by browser?

@lazysergey From what I know, the Performance result in chrome devtools doesn't format it in a text based way. The result is shown in GUI timeline and you hover over the cells and it tells what it is. So instead of showing a screenshot, I typed out the result as is.

But here's the screenshot in case

thanks, for me they're also almost identical, I don't see any layout tasks

Oh dear, it looks like changing the body's cursor with document.body.style.cursor = 'move'; invalidates the layout for the whole page. Ouch!

@waruyama do you have a test link?

@nullhook
Here is a little test case that creates an SVG with a massive amount of circles.

https://jsfiddle.net/17svhrqy/3/

1. Run it and wait until "children created" is written to the console.
2. Click on the red square and move the pointer.

If the cursor is changed on pointerdown, there is a gap of 500ms before mouse move event is triggered. Without changing the cursor (you have to comment that line), this gap is only 170ms. I am pretty sure that recalculation of the style is causing the difference.

@paulirish The Fog Creek Blog link is dead. Here is the archive.org link: https://web.archive.org/web/20180401153527/https://blog.fogcreek.com/we-spent-a-week-making-trello-boards-load-extremely-fast-heres-how-we-did-it/

I've started a little repository to reproduce these issues in order to learn from them. Maybe we can start to collect best practices there and see if some issues are resolved in the future.

https://github.com/htho/what-forces-layout-reflow

Is there any way to avoid relayout due to changing text in a text node? I've tried many variations of fixed width/height, overflow, contain, monospaced fonts, all to no avail. I would even settle for being to limit the layout to the parent node, but right now it always seems to leak to the whole document...I'm literally considering using canvas for rendering text which just seems so wrong

@skortchmark9 thank you! I tried with an embedded SVG first, but it didn't do the trick. Using canvas was the only solution that worked. Anyway if someone knows some alternative I'd like to hear about it.

Assignment to elem.style.transform value, invokes paint stage, also assignment to elem.style.left value, invokes paint stage twice. But if you will create property will-change, you don't see paint stage nowhere.

A sneaky one I just found in the canvas2D API which currently only concerns Firefox:
setting ctx.fillStyle, ctx.strokeStyle or ctx.shadowColor will force a relayout there, when the associated canvas element is attached to the DOM.
This is because these values can accept a value of "currentColor", so the browser has to perform a recalc to get the computed color. Chrome and Safari have a bug where they only get that value from the canvas element's style attribute, so they don't need to trigger the recalc there. However their behavior is against the specs, so it may change in the future (or maybe the specs will change).

Test page: https://jsfiddle.net/sxe36Lk1/