The MediaTrackConstraints dictionary's displaySurface property is a ConstrainDOMString describing the requested or mandatory constraints placed upon the value of the displaySurface constrainable property.
This is used to specify the type or types of display surfaces which getDisplayMedia() will let the user select among for sharing purposes.
If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.displaySurface as returned by a call to MediaDevices.getSupportedConstraints(). However, typically this is unnecessary since browsers will ignore any constraints they're unfamiliar with.
A ConstrainDOMString which specifies the type of display surface that's being captured. This value does not affect the list of display sources in the browser's user interface.
See How constraints are defined in Capabilities, constraints, and settings for an explanation of how to define constraints.
You can check the setting selected by the user agent after the display media has been created by getDisplayMedia() by calling getSettings() on the display media's video MediaStreamTrack, then checking the value of the returned MediaTrackSettings object's displaySurface object.
For example, if your app needs to know that the surface being shared is a monitor or application—meaning that there's possibly a non-content backdrop—it can use code similar to this:
let mayHaveBackdropFlag = false;
let displaySurface = displayStream.getVideoTracks()[0].getSettings().displaySurface;
if (displaySurface === "monitor" || displaySurface ==="application") {
mayHaveBackdropFlag = true;
}
Following this code, mayHaveBackdrop is true if the display surface contained in the stream is of type monitor or application; either of these may have non-content backdrop areas. Later code can use this flag to determine whether or not to perform special processing, such as to remove or replace the backdrop, or to "cut" the individual display areas out of the received frames of video.
Here are some example constraints objects for getDisplayMedia() that make use of the displaySurface property. In addition, see Example: Constraint exerciser in Capabilities, constraints, and settings for a complete example showing how constraints are used.
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