Started game as normal using the launcher with "latest snapshot" selected (19w35a).

Upon launching to fullscreen Minecraft game window the secondary monitor will mirror the main monitor. The mirrored version is not interactable. Trying to click within it will result in Minecraft minimizing and system behaves as though you were instead clicking on whatever app is behind it.

Troubleshooting attempted:

Exited Minecraft, & Restarted. Issue persisted.

Restarted Windows, issue persisted.

Exiting fullscreen will remove the mirrored window however returning to fullscreen will bring back the mirror.