The sourceCapabilities read-only property of the UIEvent interface returns
an instance of the InputDeviceCapabilities interface which provides
information about the physical device responsible for generating a touch event. If no
input device was responsible for the event, it returns null.
When a single user interaction with an input device generates a series of different
input events, the sourceCapabilities property for all of them will point to
the same instance of InputDeviceCapabilities. For example, when a user
lifts their finger off of a touchscreen, several UIEvents may be generated including
touchend, mousedown, click, and
focus. All of these events must have the same
sourceCapabilities representing the touchscreen.
A device is considered "responsible" for an event only when that interaction is part of
the abstraction provided by the web platform. For example, many user agents allow a
window to be resized with a mouse or a keyboard, but this detail is not exposed to the
web platform in any way, and so the sourceCapabilities of a resize event will typically
be null.
