Open Radar

 
Summary:
Provide a descriptive summary of the issue.
When putting computer A into target display mode over thunderbolt it shows brightness set to zero, and doesn't allow it to be adjusted. 

Steps to Reproduce:
In numbered format, detail the exact steps taken to produce the bug.
Put computer A into target display mode using command + f2.

Expected Results:
Describe what you expected to happen when you executed the steps above.
Shows normal brightness, allows brightness to be controlled.

Actual Results:
Explain what actually occurred when steps above were executed.
Brightness on display A set to zero. No brightness control on A.

Regression:
Describe circumstances where the problem occurs or does not occur, such as software versions and/or hardware configurations.
If I put computer B into sleep, and then log back into computer A and adjust the brightness, then sometimes when I go back into target display mode I will be able to adjust A's brightness. This solution only works intermittently.

Notes:
Provide additional information, such as references to related problems, workarounds and relevant attachments.