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.

Currently in Chromium: entering text input into a text area causes Layout:

https://bugs.chromium.org/p/chromium/issues/detail?id=1116001&q=textarea%20performance&can=2

look at this: https://codepen.io/chriscoyier/pen/EyRroJ

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/

Weird how changing the focus triggers a style recalculation, like I don't even have any :focus or :focus-visible selectors, what's a style recalculation good for in this case? It looks like wasted time to me.

@fabiospampinato it's the other way around. These getters and methods do force a reflow because they need an updated layout to work well, so it's not the fact that you have a :focus selector that matters¹, but that the focus() method needs an up to date layout.
In the particular case of element.focus() for instance, it needs to check if the element is being-rendered, otherwise it's not a focusable-area, and thus can't be focused.

Footnotes

1. Might be noted that the default UA stylesheet does have :focus-visible rule. ↩

@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

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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...).

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 elements...

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

Frustrating that it seems like even overriding ctx.direction to 'ltr' or 'rtl' doesn't save us from the reflow on .fillText(). https://jsfiddle.net/c92jmnz3/

If I tell my CanvasRenderingContext2D to use direction='ltr', shouldn't it no longer need to check styles?

Hmm, seems like there's at least one more reason why Blink unconditionally triggers a reflow on fillText():

accessFont needs the style to be up to date, but updating style can cause
script to run, (e.g. due to autofocus) which can free the canvas (set
size to 0, for example), so update style before grabbing the PaintCanvas.

https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.cc;l=3131-3135;drc=c0265133106c7647e90f9aaa4377d28190b1a6a9

Can you guys help me troubleshoot why content-visibility: auto; is not working on a particular page? I'm applying this to <tr> and usually rows that outside of the viewable area should have a "Descendants are skipped due to content-visibility" tooltip, but I'm still not seeing it. I'm checking out the Performance tab, but I can't figure out exactly what is triggering the repaint that prevents this from working. I double-checked to make sure the page is not using any of the things listed in what-forces-layout.md.

Anyone else seen reading width of an HTMLImageElement causing a style reclaculation?

Github is broken so I can't edit comments or add pictures but it seems like element.getAttribute("width") needs to be used instead, width returns the same thing as offsetWidth which is the measured dimensions of the image. Maybe this should be added to the list?

I guess computedStyleMap() forces reflow, but maybe not since it doesn't resolve values?

@oliviertassinari like getComputedStyle the call itself is free, it's calling get() on it that will cause a reflow. And yes, while the values aren't the "resolved" ones they still force a layout, at least in current Chrome's implementation: https://jsfiddle.net/sempzykg/ I guess they need it even to be able to perform the cascading.

@Kaiido Amazing, thanks for writing that test case!