I successfully mirrored my internal MacBook Pro display to a 3rd gen Apple TV. The resolution shifted as expected, audio in the connected TV works, video looks fine, but when I try to take the cursor past about 40% to the right I seem to hit an invisible wall.
This makes it impossible to put some videos full screen, as I can't reach the controls for this, and otherwise makes it awkward to operate stuff.
Is this something I should have known about, like a well-known limitation or a bug in macOS Sierra 10.12?