UserAgent: Mozilla/5.0 (X11; CrOS x86_64 8872.73.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.103 Safari/537.36
Platform: 8872.73.0 (Official Build) stable-channel celes

Steps to reproduce the problem:
1. Turn off all devices
2. Connect external monitor via HDMI cable
3. Power on monitor, then chromebook
4. Use chromebook as normal
5. External screen goes blank, chromebook screen still works fine. Changing to extended screen with external monitor as primary display does not help.
6. To get external display working again repeat steps one through four.

What is the expected behavior?
External screen is a mirror of chromebook screen.

What went wrong?
External screen is blank...

Did this work before? No 

Chrome version: 55.0.2883.103  Channel: stable
OS Version: 8872.73.0
Flash Version: Shockwave Flash 24.0 r0