Visual Viewport API

tl;dr

We propose adding a visualViewport object on window that contains the properties of the visual viewport. We're incubating this idea via the WICG in order to try to make incremental progress on the long-standing problem of exposing features like pinch-zoom to web developers in a rational way. We are working with the CSSWG to eventually get these ideas into the relevant specs as first-class features of the web platform.

Update: The window.visualViewport API has shipped in Chrome M61 (Sept. 2017). Follow crbug issue 635031 for details.

Spec

Draft Spec

~~Draft Spec~~

Now merged upstream to CSSOM-View

Background

The mobile web contains two viewports, the Layout and Visual viewport. The Layout viewport is what a page lays out its elements into(*) and the Visual viewport is what is actually visible on the screen. When the user pinch-zooms into the page, the visual viewport shrinks but the layout viewport is unchanged. UI like the on-screen keyboard (OSK) can also shrink the visual viewport without affecting the layout viewport. See this demo to visualize the two viewports. This isn't specified anywhere and implementations vary greatly between browsers.

Currently, several CSSOM scroll properties are relative to the visual viewport (see this for list). Again, there is no spec governing this, but this is how browsers have it implemented today. With this implementation, the dimensions of the visual viewport can be easily determined (For example, window.innerHeight = visual viewport height). However, all other coordinates are generally relative to the layout viewport (e.g. getBoundingClientRects, elementFromPoint, event coordinates, etc.). Having these APIs be mixed is arbitrary and confusing.

This confusion has caused many desktop sites to break when pinch-zoomed or when showing the OSK (see this bug for examples). This is because mobile browsers added new semantics to existing properties, expecting they'd to be invisible to desktop browsers. This becomes a problem as the lines between mobile and desktop blur and features like on-screen keyboard and pinch-zoom make their way to desktops, or when accessing desktop pages from mobile devices.

(*) - This isn't strictly true. In Chrome, the layout viewport is actually the "viewport at minimum scale". While on most well behaving pages this is the box that the page lays out into (i.e. the initial containing block), extra-wide elements or an explicit minimum-scale can change this. More specifically, the layout viewport is what position: fixed elements attach to.

Proposed Plan

We believe the best way forward is to change those remaining CSSOM scroll properties to be relative to the layout viewport. In fact, Chrome did this in M48 but, due to developer feedback, this change was reverted in M49. There was more reliance on this than anticipated.

In order to make this transition we propose adding a new explicit API for the visual viewport. With an explicit API, we could once again change the CSSOM scroll properties to be relative to the layout viewport. This change would make sure existing desktop sites continue to function correctly as new UI features are added. At the same time, it would allow authors to use and customize those features where needed.

The new API is also easy to feature detect and polyfilling this behavior should be fairly straightforward.

Proposed API (v1)

Add a visualViewport object on window.

visualViewport = {
    double offsetLeft; // Relative to the layout viewport
    double offsetTop; // and read-only.

    double pageLeft;  // Relative to the document
    double pageTop;  // and read-only.

    double width;  // Read-only and excludes the scrollbars
    double height; // if present. These values give the number
                   // of CSS pixels visible in the visual viewport.
                   // i.e. they shrink as the user zooms in.

    double scale;     // Read-only. The scaling factor applied to
                      // the visual viewport relative to the `ideal
                      // viewport` (size at width=device-width). This
                      // is the same scale as used in the viewport
                      // <meta> tag.
    
    FrozenArray<DOMRect> segments; // Read-only. Returns an array of 
                                   // DOMRects that represent the dimensions 
                                   // of each existing viewport segment.

}

Fire a scroll event against window.visualViewport whenever the offsetLeft or offsetTop attributes change.
Fire a resize event against window.visualViewport whenever the width or height attributes change.
The viewport segments property is currently in development and experimental. Please view the segments explainer for more details.

Example

Here's how an author might use this API to simulate position: device-fixed, which fixes elements to the visual viewport.

Live example from below

<meta name="viewport" content="width=device-width">
<style>
    #layoutViewport {
        position: fixed;
        width: 100%;
        height: 100%;
        visibility: hidden;
    }
    #bottombar {
        position: fixed;
        left: 0px;
        right: 0px;
        bottom: 0px;
        background-color: red;
        transform-origin: left bottom;
        transform: translate(0px, 0px) scale(1);
    }
    #forcescrolling {
        width: 100px;
        height: 2000px;
        background-color: green;
    }
</style>

<body>
    <div id="bottombar">This stays stuck to the visual viewport</div>
    <div id="forcescrolling"></div>
    <div id="layoutViewport"></div>
</body>

<script>
    var bottomBar = document.getElementById('bottombar');
    var viewport = window.visualViewport;
    function viewportHandler() {
        var layoutViewport = document.getElementById('layoutViewport');

        // Since the bar is position: fixed we need to offset it by the visual
        // viewport's offset from the layout viewport origin.
        var offsetLeft = viewport.offsetLeft;
        var offsetTop = viewport.height
                    - layoutViewport.getBoundingClientRect().height
                    + viewport.offsetTop;

        // You could also do this by setting style.left and style.top if you
        // use width: 100% instead.
        bottomBar.style.transform = 'translate(' +
                                    offsetLeft + 'px,' +
                                    offsetTop + 'px) ' +
                                    'scale(' + 1/viewport.scale + ')'
    }
    window.visualViewport.addEventListener('scroll', viewportHandler);
    window.visualViewport.addEventListener('resize', viewportHandler);
</script>

Other Examples

Here's a few other examples you can try out on Chrome Canary today. Be sure to turn on the following flags:

chrome://flags/#enable-experimental-web-platform-features (Enable window.visualViewport)
chrome://flags/#inert-visual-viewport (Makes window.scrollX|innerWidth and others refer to layout viewport)
chrome://flags/#enable-osk-overscroll (Makes keyboard resize visual viewport only)

Polyfill

TODO: Doesn't work on iOS Safari yet. We've added a rudimentary polyfill that should work across browsers, albeit with worse performance properties (requires polling and ugly hacks). The polyfill itself is visualViewport.js and you can see two examples that use it in the same directory:

WICG/visual-viewport

WICG

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