Use the mentalray_max.ini file to disable or enable display of white brackets around buckets while rendering an image with the mental ray renderer.
When this option is active, 3ds Max displays white selection brackets at the corners of the bucket currently being rendered.
To disable bucket bracket display:
This file is in the folder C:\Users\<userid>\AppData\Local\Autodesk\3ds Max\2016 - 64bit\ENU\en-US\plugcfg\.
[Preferences]
ShowRenderBracket=0
To enable mental ray extension display:
This file is in the folder C:\Users\<userid>\AppData\Local\Autodesk\3ds Max\2016 - 64bit\ENU\en-US\plugcfg\.