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diff --git a/docs/performance/scroll-linked_effects.md b/docs/performance/scroll-linked_effects.md new file mode 100644 index 0000000000..90d3c33ed1 --- /dev/null +++ b/docs/performance/scroll-linked_effects.md @@ -0,0 +1,177 @@ +# Scroll-linked effects + +The definition of a scroll-linked effect is an effect implemented on a +webpage where something changes based on the scroll position, for +example updating a positioning property with the aim of producing a +parallax scrolling effect. This article discusses scroll-linked effects, +their effect on performance, related tools, and possible mitigation +techniques. + +## Scrolling effects explained + +Often scrolling effects are implemented by listening for the `scroll` +event and then updating elements on the page in some way (usually the +CSS +[`position`]((https://developer.mozilla.org/en-US/docs/Web/CSS/position "The position CSS property sets how an element is positioned in a document. The top, right, bottom, and left properties determine the final location of positioned elements.") +or +[`transform`](https://developer.mozilla.org/en-US/docs/Web/CSS/transform "The transform CSS property lets you rotate, scale, skew, or translate an element. It modifies the coordinate space of the CSS visual formatting model.") +property.) You can find a sampling of such effects at [CSS Scroll API: +Use +Cases](https://github.com/RByers/css-houdini-drafts/blob/master/css-scroll-api/UseCases.md). + +These effects work well in browsers where the scrolling is done +synchronously on the browser's main thread. However, most browsers now +support some sort of asynchronous scrolling in order to provide a +consistent 60 frames per second experience to the user. In the +asynchronous scrolling model, the visual scroll position is updated in +the compositor thread and is visible to the user before the `scroll` +event is updated in the DOM and fired on the main thread. This means +that the effects implemented will lag a little bit behind what the user +sees the scroll position to be. This can cause the effect to be laggy, +janky, or jittery --- in short, something we want to avoid. + +Below are a couple of examples of effects that would not work well with +asynchronous scrolling, along with equivalent versions that would work +well: + +### Example 1: Sticky positioning + +Here is an implementation of a sticky-positioning effect, where the +\"toolbar\" div will stick to the top of the screen as you scroll down. + +```html +<body style="height: 5000px" onscroll="document.getElementById('toolbar').style.top = Math.max(100, window.scrollY) + 'px'"> + <div id="toolbar" style="position: absolute; top: 100px; width: 100px; height: 20px; background-color: green"></div> +</body> +``` + +This implementation of sticky positioning relies on the scroll event +listener to reposition the "toolbar" div. As the scroll event listener +runs in the JavaScript on the browser's main thread, it will be +asynchronous relative to the user-visible scrolling. Therefore, with +asynchronous scrolling, the event handler will be delayed relative to +the user-visible scroll, and so the div will not stay visually fixed as +intended. Instead, it will move with the user's scrolling, and then +\"snap\" back into position when the scroll event handler runs. This +constant moving and snapping will result in a jittery visual effect. One +way to implement this without the scroll event listener is to use the +CSS property designed for this purpose: + +```html +<body style="height: 5000px"> + <div id="toolbar" style="position: sticky; top: 0px; margin-top: 100px; width: 100px; height: 20px; background-color: green"></div> +</body> +``` + +This version works well with asynchronous scrolling because position of +the \"toolbar\" div is updated by the browser as the user scrolls. + +### Example 2: Scroll snapping + +Below is an implementation of scroll snapping, where the scroll position +snaps to a particular destination when the user's scrolling stops near +that destination. + +```html +<body style="height: 5000px"> + <script> + function snap(destination) { + if (Math.abs(destination - window.scrollY) < 3) { + scrollTo(window.scrollX, destination); + } else if (Math.abs(destination - window.scrollY) < 200) { + scrollTo(window.scrollX, window.scrollY + ((destination - window.scrollY) / 2)); + setTimeout(snap, 20, destination); + } + } + var timeoutId = null; + addEventListener("scroll", function() { + if (timeoutId) clearTimeout(timeoutId); + timeoutId = setTimeout(snap, 200, parseInt(document.getElementById('snaptarget').style.top)); + }, true); + </script> + <div id="snaptarget" class="snaptarget" style="position: relative; top: 200px; width: 100%; height: 200px; background-color: green"></div> +</body> +``` + +In this example, there is a scroll event listener which detects if the +scroll position is within 200 pixels of the top of the \"snaptarget\" +div. If it is, then it triggers an animation to \"snap\" the scroll +position to the top of the div. As this animation is driven by +JavaScript on the browser's main thread, it can be interrupted by other +JavaScript running in other tabs or other windows. Therefore, the +animation can end up looking janky and not as smooth as intended. +Instead, using the CSS snap-points property will allow the browser to +run the animation asynchronously, providing a smooth visual effect to +the user. + +```html +<body style="height: 5000px"> + <style> + body, /* blink currently has bug that requires declaration on `body` */ + html { + scroll-snap-type: y proximity; + } + .snaptarget { + scroll-snap-align: start; + position: relative; + top: 200px; + height: 200px; + background-color: green; + } + </style> + <div class="snaptarget"></div> +</body> +``` + +This version can work smoothly in the browser even if there is +slow-running Javascript on the browser's main thread. + +### Other effects + +In many cases, scroll-linked effects can be reimplemented using CSS and +made to run on the compositor thread. However, in some cases the current +APIs offered by the browser do not allow this. In all cases, however, +Firefox will display a warning to the developer console (starting in +version 46) if it detects the presence of a scroll-linked effect on a +page. Pages that use scrolling effects without listening for scroll +events in JavaScript will not get this warning. See the [Asynchronous +scrolling in Firefox](https://staktrace.com/spout/entry.php?id=834) blog +post for some more examples of effects that can be implemented using CSS +to avoid jank. + +## Future improvements + +Going forward, we would like to support more effects in the compositor. +In order to do so, we need you (yes, you!) to tell us more about the +kinds of scroll-linked effects you are trying to implement, so that we +can find good ways to support them in the compositor. Currently there +are a few proposals for APIs that would allow such effects, and they all +have their advantages and disadvantages. The proposals currently under +consideration are: + +- [Web Animations](https://w3c.github.io/web-animations/): A new API + for precisely controlling web animations in JavaScript, with an + [additional + proposal](https://wiki.mozilla.org/Platform/Layout/Extended_Timelines) + to map scroll position to time and use that as a timeline for the + animation. +- [CompositorWorker](https://docs.google.com/document/d/18GGuTRGnafai17PDWjCHHAvFRsCfYUDYsi720sVPkws/edit?pli=1#heading=h.iy9r1phg1ux4): + Allows JavaScript to be run on the compositor thread in small + chunks, provided it doesn't cause the framerate to drop. +- [Scroll + Customization](https://docs.google.com/document/d/1VnvAqeWFG9JFZfgG5evBqrLGDZYRE5w6G5jEDORekPY/edit?pli=1): + Introduces a new API for content to dictate how a scroll delta is + applied and consumed. As of this writing, Mozilla does not plan to + support this proposal, but it is included for completeness. + +### Call to action + +If you have thoughts or opinions on: + +- Any of the above proposals in the context of scroll-linked effects. +- Scroll-linked effects you are trying to implement. +- Any other related issues or ideas. + +Please get in touch with us! You can join the discussion on the +[public-houdini](https://lists.w3.org/Archives/Public/public-houdini/) +mailing list. |