As the title says, this issue is application specific. 

In Assembly Design (same part, assembly space) I'm unable to orbit using the middle mouse button, and cannot scroll zoom with the wheel. 

In Part Design, the usual interaction methods work just fine.  I can rotate the part in any axis and zoom with the scroll wheel.  

I've tried updating, reset all preferences, and restarting the apps.  No change.  One app works properly where the other does not.  

What did I do?  What can be done?