SEGMENTS-EXPLAINER.md

Viewport Segments Property

Daniel Libby, Stephanie Stimac

Related explainers:

Name	Link
Screen Fold API	Explainer, Unofficial Draft Spec
Multi-Screen Window Placement API	Explainer, Draft Community Group Report
Visual Viewport API	Draft Community Group Report, MDN

Motivation:

Web developers targeting foldable devices want to be able to effectively lay out the content in a window that spans multiple displays. However, the web platform does not yet provide the necessary primitives for building layouts that are optimized for foldable experiences. Developers may be able to solve this by taking a hard dependency on a specific device hardware parameters - an approach that is fragile, not scalable, and requires work duplication for each new device.

Current problems:

More specific challenges we've heard from our internal product teams that were exploring building experiences for this emerging classes of devices include:

• Hardware differences: Devices could be seamless (e.g. Samsung Galaxy Fold) or have a seam (e.g. Microsoft Surface Duo or ZTE Axon M). In the latter case developers might want to take it into account or intentionally ignore depending on scenario;
• Folding capabilities, state: the fold area could be safe or unsafe region to present content;
• Future-proofing: Ideally developers would want a somewhat stable way to target this class of devices without having to rely on specific device hardware parameters.

Complementary existing proposals:

Before discussing the solution proposal - let's overview existing proposals that are relevant and applicable to the problem space. As matter of principle we should generally avoid creating redundant concepts if we can reuse existing platform APIs and capabilities.

• Presentation API is solving the problem of a effective use of a secondary screen and likely won't attempt to solve challenges outlined above that are specific to devices where a window can span separate physical displays. This would likely still be a separate problem for foldables

• Screen Enumeration API Explainer provides information about the physical screen configuration. Web developers might be able to leverage that on foldables, but would need to do extra effort to correlate that information with window parameters. Some concrete examples on why a special purpose API might be useful in addition to Screen Enumeration:

  • Getting adjacency information about spanning window regions to lay out content in several areas in logical way for a device;
  • Getting inner window dimensions that account for application frame, OS UI elements, etc.

• Window Placement API Explainer is useful in multi-window scenarios on multiple screen devices, but does not target scenarios in which the hosting application (i.e. browser) has a single window which spans multiple displays. In this case, the developer may not wish to open new windows - just hints to help lay out things properly and take advantage of the physical partitioning of the available layout space.

Additionally, while not a solution in the same sense, a "[css-media-queries] Foldables support and enablement" issue discusses the problem space and outlines some details and touches upon outlined issues.

Proposal: Viewport Segments Property

A summary of the concepts from the other proposals:

• Display - the logical representation of an physical monitor.
• Screen - the aggregate 2D space occupied by all the connected displays.

We propose a new concept of Viewport Segments, which represent the regions of the window that reside on separate (adjacent) displays. Viewport Segment dimensions are expressed in CSS pixels and will be exposed via a JavaScript API that allows developers to enumerate segments where logically separate pieces of content can be placed.

This proposal is primarily aimed at reactive scenarios, where an application wants to take advantage of the fact that it spans multiple displays, by virtue of the user/window manager placing it in that state. It is not designed for scenarios of proactively placing content in a separate top-level browsing context on the various displays available (this would fall under the Window Placement API or Presentation API). Note that given the Screen Enumeration API and existing primitives on the Web, it is possible to write JavaScript code that intersects the rectangles of the Display and window, while taking into account devicePixelRatio in order to compute the interesting layout regions of a window spanned across displays. However this may not correctly handle corner cases of future device form factors, and thus this proposal tries to centralize access to "here are the interesting parts of the screen a developer can target or consider for presenting content" as a practical starting point.

interface VisualViewport : EventTarget {
	readonly attribute FrozenArray<DOMRect> segments;
}

The value returned from querying visualViewport.segments will be an array of DOMRects and based on the data returned for each "viewport segment", developers will be able to infer the number of hinges available as well as the hinge orientation. When visualViewport.segments is queried on a desktop, this will return null since there is only one viewport segment.

A user may at any point take the browser window out of spanning mode and place it on one of the screens or vice-versa, in those cases the window resize event will fire and authors can query and get the number of available viewport segments.

This proposal doesn't aim to substitute existing APIs — the proposed development model can be summarized as requesting current viewport segments on interesting events and adjusting to the new presentation environment. There are no additional lifecycle proposals - the viewport segments are immutable and developers would request them upon common sense events (e.g. orientationchange, resize). It also doesn't suggest how developers would use viewport segments to position, scale and orient content - in practical explorations developers used viewport segments to select the best declarative layout, not to modify layouts in script, but either would be possible.

Security and Privacy

APIs avalibility in iframe context

segments will return null when called from within an iframe context.

Examples of user experiences and solution outlines that can leverage two screens:

Let's take a look at a few practical examples of the scenarios above and how viewport segments would allow to resolve them for better user experience. In each case we'll start with some existing scenario and complicate it to provide opportunity to apply the proposal.

A map application that presents a map on one window segment and search results on another

JavaScript solution outline:

const segments = window.visualViewport.segments;

if( segments.length > 1 ) {
	// now we know the device is a foldable
	// it's recommended to test whether segments[0].width === segments[1].width
	// and we can update CSS classes in our layout as appropriate 
	document.body.classList.add('is-foldable');
	document.querySelector('.map').classList.add('flex-one-half');
	document.querySelector('.locations-list').classList.add('flex-one-half');
}

Reacting to map application resize/spanning state change

JavaScript solution outline:

window.onresize = function() {
	const segments = window.visualViewport.segments;
	console.log(segments.length) // 1
}