Chrome Version       : 55.0.2883.7
OS Version: 8872.6.0
URLs (if applicable) :
Other browsers tested:
  Add OK or FAIL after other browsers where you have tested this issue:
     Safari 5:
  Firefox 4.x:
     IE 7/8/9:

What steps will reproduce the problem?
1. Close lid of Chromebook
2. Attach powered-down external display via HTMI cable (with HDMI<>USB-C adaptor) 
3. List lid on Chromebook
4. Power on the display

What is the expected result?
The screen is extended to the external display

What happens instead of that?
The Chromebook continually polls through negotiation attempts but rarely extends

Please provide any additional information below. Attach a screenshot if
possible.

This is the odd thing - if I sign in while this is going on and try to provide feedback, then as the feedback window appears the screen is also extended! 

The screen is a Dell S2409W.

F(eedback sent 12:40 BST)

- Mike

UserAgentString: Mozilla/5.0 (X11; CrOS x86_64 8872.6.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.7 Safari/537.36