Skip to content

Instantly share code, notes, and snippets.

What would you like to do?
What forces layout/reflow. The comprehensive list.

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.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.scrollingElement only forces style
  • document.elementFromPoint

Forms: Setting selection + focus

  • inputElem.focus()

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()


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!


  • 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.

More on forced layout

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

Copy link

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

Copy link

Copy link

aquaductape commented Mar 2, 2021

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

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
Here's the source

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

Copy link

mfbx9da4 commented Mar 29, 2021

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

Copy link

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?

Copy link

aquaductape commented May 15, 2021

@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

Copy link

lazysergey commented May 17, 2021

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

Screenshot 2021-05-17 at 8 44 02 AM

Screenshot 2021-05-17 at 8 41 54 AM

Copy link

waruyama commented Jul 1, 2021

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

Copy link

nullhook commented Jul 1, 2021

@waruyama do you have a test link?

Copy link

waruyama commented Jul 2, 2021

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

  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.

Copy link

mootari commented Aug 8, 2021

Copy link

htho commented Sep 6, 2021

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.

Copy link

matonga commented Nov 25, 2021

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.

Copy link

dSalieri commented Feb 11, 2022

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

Copy link

Kaiido commented Mar 11, 2022

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:

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment