The "mouse click + drag" selection box for slaves/bases/drones stops working if you open the menu for a slave and use the "Transfer Ownership" feature. After then, the selection box stops working forever, even if you close the window and all. Restarting the client fixes it.

Also, I just found another fix that may help debug the code. If I type and say something in any chat, the selection box works afterwards until you do the Transfer Ownership menu option again.