Primary display randomly changes I'm using a MacBook Pro (Late 2011) connected to two Thunderbolt displays. The displays are chained together, and one Thunderbolt connection is made to the MacBook Pro.

I prefer to have the left monitor function as the primary display (I want the dock to live on the left). Using the Display System Preference Pane, I can set this configuration with no issues.

Every morning, I connect the displays to the MacBook Pro and I disconnect every evening. 

Some days, seemingly at random, the display configuration is changed and the right monitor has the dock and menu bar instead of the left.

Is there any way to ensure the display configuration remains the same?
This worked for me, credit to this answer:

https://superuser.com/a/1220526/454133


  Play around with moving the "top" bar between screens in the Arrangement window. Assign it to different screens and you will notice it will solve your issue once you find the "right" screen.


So, monkeying with the menu bar in the arrangement settings will reset your displays proper. I even got screenshots.