Microsoft today released Windows 10 SDK Preview Build 17704 to developers. This SDK can be used with the recently released Windows 10 Insider Build 17704 or later. This updated SDK comes with bug fixes and changes to the API surface area.
Microsoft has included API support for hinge angle sensor in this SDK build. This shows that Microsoft is currently actively developing ways the OS can respond to the hinge state of a 2-panel dual-screen device. You can read more about it
here.

You can find the list of new API additions
here. The updated Preview SDK can be downloaded
here.
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