State of the Art (in Chrome)

When an application calls getDisplayMedia(), and the user chooses to capture a tab, Chrome displays an tab-capture infobar right below the omnibar. When a tab other than the capturing/captured tab is focused, the only button in this infobar is [Stop sharing].

If using the Extension API for screen-sharing, another button is exposed - [Share this tab instead].

Pressing this button changes the target of the tab-capture to the other tab.

Innovation

We now expose this functionality to the Web at large. Web applications can control whether this button is shown by specifying a value to DisplayMediaStreamConstraints.surfaceSwitching.

The relevant spec-change is here.
The resulting text in the spec is here.

Motivation

The [Share this tab instead] button allows users to seamlessly switch which tab they’re sharing, without having to select the video-conferencing tab again (1), click a button to initiate getDisplayMedia() again (2), and selecting a new tab out of a long list of tabs again (3).

This behavior is exposed conditionally because not all Web applications are able to handle this behavior. Most notably, applications centered around self-capture and cropping might have trouble. For a more elaborate discussion, see this GitHub thread.

Activation

const stream = await navigator.mediaDevices.getDisplayMedia({
  video: true,
  surfaceSwitching: "include"  // Or exclude.
});

Common Questions

1. What if my application does not specify an explicit value?

Chrome will try to guess what’s best for your application. This guess may change between milestones. Initially:

If the user chooses the current tab, the [Share this tab instead] button will not be exposed.
If the user chooses any other tab, the [Share this tab instead] button will be exposed.

2. What about other surfaces? Windows? Monitors?

At the moment, Chrome only supports fast-switching between shared tabs. In the future, hopefully more.

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