Seen on Win10 57.0.2987.88 beta
Does not reproduce on Chrome OS

Steps to reproduce:
1. Open a browser window
2. Longpress on the browser frame (empty space to the right of the new tab button but to the left of the window controls)
3. Wait until the longpress timer fires

What happens:
Nothing

What should happen instead:
The browser's context menu should appear

Note: The context menu appears upon release of the finger.

Note: When the same steps are taken on Chrome OS, the context menu appears when the longpress timer fires, before the finger is lifted off the screen.