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.
• 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

---

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

@Kaiido that makes a lot of sense 🤔 thank you!

Can you please write the test? Unfortunately I did not see that it does not pass through

…

________________________________ Von: Fabio Spampinato ***@***.***> Gesendet: Freitag, 28. April 2023 12:50 An: fabiospampinato ***@***.***> Cc: Manual ***@***.***> Betreff: EXTERNAL: Re: paulirish/what-forces-layout.md CAUTION! External Email. Do not click links or open attachments unless you recognize the sender and are sure the content is safe. If you think this is a phishing email, please report it by using the "Phish Alert Report" button in Outlook. @fabiospampinato commented on this gist.

________________________________ @Kaiido<https://github.com/Kaiido> that makes a lot of sense 🤔 thank you! — Reply to this email directly, view it on GitHub<https://gist.github.com/paulirish/5d52fb081b3570c81e3a#gistcomment-4551750> or unsubscribe<https://github.com/notifications/unsubscribe-auth/AB4BF5CX64B3DD6QY34TZIDXDOOFZBFKMF2HI4TJMJ2XIZLTSKBKK5TBNR2WLJDHNFZXJJDOMFWWLK3UNBZGKYLEL52HS4DFQKSXMYLMOVS2I5DSOVS2I3TBNVS3W5DIOJSWCZC7OBQXE5DJMNUXAYLOORPWCY3UNF3GS5DZVRZXKYTKMVRXIX3UPFYGLK2HNFZXIQ3PNVWWK3TUUZ2G64DJMNZZDAVEOR4XAZNEM5UXG5FFOZQWY5LFVAZDMNBVHAYTGONHORZGSZ3HMVZKMY3SMVQXIZI>. You are receiving this email because you are subscribed to this thread. Triage notifications on the go with GitHub Mobile for iOS<https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android<https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>. UNRESTRICTED

The reason window.matchMedia is not on this list is because window.matchMedia("(min-width: ...)") does not depend on whether scrollbars are visible:

I believe that the list is correct and matchMedia does not trigger a reflow. All media queries have been carefully designed to not depend on page content. For example, width has been specced as viewport width including the scrollbar so that the result doesn't change regardless of whether there is a scrollbar or not.

A few notes on CSS Triggers

That site (which seems to be hosted by a squatter now?) covers what operations are required as a result of setting/changing a given CSS value. Basically, what work has to happen if you're changing just 1 style property every rAF tick.
"trigger" isn't a great name, but essentially it's poking at: what is invalidated (and thus needs to be recomputed) when style property X is changed?

For example:

• setting el.style.opacity in a rAF loop doesn't invalidate the layout or paint, so on each rAF tick, we'd see Style then Composite. (skipping layout & paint)
• meanwhile: setting el.style.left in a rAF loop affects layout as well. (And, as a rule, all layout changes need to be painted). So on each rAF tick we see all 4 operations: style,layout,paint,composite.

So while csstriggers is/was about "what is the impact of setting a style value?", the gist above is about "what is the impact of getting a value".

(tbh generally I think the csstriggers data is mostly a distraction/oversimplification and you should just use the devtools performance panel instead)

A couple of new triggers I found, related to the Canvas2D API and my previous comment.

Another sneaky setter, that concerns both Firefox and Chrome: setting the Canvas2D .filter will force a reflow. https://jsfiddle.net/bqfm4agh/ I suppose this is because this property can also have a currentColor component (in drop-shadow()) and that they'll gather the computed styles every time.

Then, the Canvas2D API has a direction attribute, its default value is inherit, so it's taken from the computed style of the target canvas and thus everything that will get this attribute will need to trigger a reflow for connected <canvas> elements...

In Chrome the ctx.direction getter will trigger a reflow, that one is not very surprising. https://jsfiddle.net/mhn4y60c/

In Firefox the direction getter did not trigger a reflow, but that's because they computed it when we called getContext("2d") o_O https://jsfiddle.net/pdyc6v49/ That's certainly more surprising.

And then, in every browser, calling .fillText() or .strokeText() will also trigger a reflow. https://jsfiddle.net/80rgLymn/ You get it, they need the computed direction value for drawing the text, but while it makes sense when you know it, it's definitely not obvious (I discovered it very recently, while I'm interested on this kinda stuff for years now, and did work intensively on this API...).