Asynchronous scrolling in Firefox

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Posted by: stak
Tags: mozilla
Posted on: 2015-11-30 13:32:51

In the Firefox family of products, we've had asynchronous scrolling (aka async pan/zoom, aka APZ, aka compositor-thread scrolling) in Firefox OS and Firefox for Android for a while - even though they had different implementations, with different behaviors. We are now in the process of taking the Firefox OS implementation and bringing it to all our other platforms - including desktop and Android. After much hard work by many people, including but not limited to :botond, :dvander, :mattwoodrow, :mstange, :rbarker, :roc, :snorp, and :tn, we finally have APZ enabled on the nightly channel for both desktop and Android. We're working hard on fixing outstanding bugs and getting the quality up before we let it ride the trains out to DevEdition, Beta, and the release channel.

If you want to try it on desktop, note that APZ requires e10s to be enabled, and is currently only enabled for mousewheel/trackpad scrolling. We do have plans to implement it for other input types as well, although that may not happen in the initial release.

Although getting the basic machinery working took some effort, we're now mostly done with that and are facing a different but equally challenging aspect of this change - the fallout on web content. Modern web pages have access to many different APIs via JS and CSS, and implement many interesting scroll-linked effects, often triggered by the scroll event or driven by a loop on the main thread. With APZ, these approaches don't work quite so well because inherently the user-visible scrolling is async from the main thread where JS runs, and we generally avoid blocking the compositor on main-thread JS. This can result in jank or jitter for some of these effects, even though the main page scrolling itself remains smooth. I picked a few of the simpler scroll effects to discuss in a bit more detail below - not a comprehensive list by any means, but hopefully enough to help you get a feel for some of the nuances here.

Smooth scrolling

Smooth scrolling - that is, animating the scroll to a particular scroll offset - is something that is fairly common on web pages. Many pages do this using a JS loop to animate the scroll position. Without taking advantage of APZ, this will still work, but can result in less-than-optimal smoothness and framerate, because the main thread can be busy with doing other things.

Since Firefox 36, we've had support for the scroll-behavior CSS property which allows content to achieve the same effect without the JS loop. Our implementation for scroll-behavior without APZ enabled still runs on the main thread, though, and so can still end up being janky if the main thread is busy. With APZ enabled, the scroll-behavior implementation triggers the scroll animation on the compositor thread, so it should be smooth regardless of load on the main thread. Polyfills for scroll-behavior or old-school implementations in JS will remain synchronous, so for best performance we recommend switching to the CSS property where possible. That way as APZ rolls out to release, you'll get the benefits automatically.

Here is a simple example page that has a spinloop to block the main thread for 500ms at a time. Without APZ, clicking on the buttons results in a very janky/abrupt scroll, but with APZ it should be smooth.


Another common paradigm seen on the web is "sticky" elements - they scroll with the page for a bit, and then turn into position:fixed elements after a point. Again, this is usually implemented with JS listening for scroll events and updating the styles on the elements based on the scroll offset. With APZ, scroll events are going to be delayed relative to what the user is seeing, since the scroll events arrive on the main thread while scrolling is happening on the compositor thread. This will result in glitches as the user scrolls.

Our recommended approach here is to use position:sticky when possible, which we have supported since Firefox 32, and which we have support for in the compositor. This CSS property allows the element to scroll normally but take on the behavior of position:fixed beyond a threshold, even with APZ enabled. This isn't supported across all browsers yet, but there are a number of polyfills available - see the resources tab on the Can I Use position:sticky page for some options.

Again, here is a simple example page that has a spinloop to block the main thread for 500ms at a time. With APZ, the JS version will be laggy but the position:sticky version should always remain in the right place.


Parallax. Oh boy. There's a lot of different ways to do this, but almost all of them rely on listening to scroll events and updating element styles based on that. For the same reasons as described in the previous section, implementations of parallax scrolling that are based on scroll events are going to be lagging behind the user's actual scroll position. Until recently, we didn't have a solution for this problem.

However, a few days ago :mattwoodrow landed compositor support for asynchronous scroll adjustments of 3D transforms, which allows a pure CSS parallax implementation to work smoothly with APZ. Keith Clark has a good writeup on how to do this, so I'm just going to point you there. All of his demo pages should scroll smoothly in Nightly with APZ enabled.

Unfortunately, it looks like this CSS-based approach may not work well across all browsers, so please make sure to test carefully if you want to try it out. Also, if you have suggestions on other methods on implementing parallax so that it doesn't rely on a responsive main thread, please let us know. For example, :mstange created one at which we should be able to support in the compositor without too much difficulty.

Other features

I know that there are other interesting scroll-linked effects that people are doing or want to do on the web, and we'd really like to support them with asynchronous scrolling. The Blink team has a bunch of different proposals for browser APIs that can help with these sorts of things, including things like CompositorWorker and scroll customization. For more information and to join the discussion on these, please see the public-houdini mailing list. We'd love to get your feedback!

(Thanks to :botond and :mstange for reading a draft of this post and providing feedback.)

Posted by Tom at 2015-12-02 03:56:50
I tried to enable APZ, but I still get lag on the demo pages, what am I doing wrong?
[ Reply to this ]
Posted by stak at 2015-12-02 08:06:05
Hard to say what you did wrong if you don't describe what you did... What platform/version of Firefox, and what did you do to enable it?
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Posted by Tom at 2015-12-02 08:28:37
I am using nightly on windows and checked if layers.async-pan-zoom.enabled was enabled. I guess that is the way to enable it (maybe next time also add it in your blog how to enable it)? Then I went to the demo page, but when clicking the scroll buttons, there is a big lag.
[ Reply to this ]
Posted by stak at 2015-12-02 09:59:55
So yes, nightly on windows should have APZ enabled by default and should work provided you haven't disabled e10s. Note that on the demo page, there will be a lag when starting the scroll (i.e. it won't happen right when you click) because the click event is still processed on the main thread. However, once the scroll starts, the animation should be smooth and shouldn't just "jump" to the end. Scrolling with the mousewheel should not have the delay since we won't wait for the main thread there.
[ Reply to this ]
Posted by Arriba at 2016-06-16 03:54:45
I think you want to help people that do not want help.
Just put a scrolling css option that would switch scrolling mode from synchronous to asynchronous, and everyone will be happy.
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