Cancelling a merge causes Sourcetree to become unusable

When I try to merge one branch into another, and then I click 'Cancel' - I can no longer register clicks ANYWHERE in SourceTree.

I just get the sound that indicates I'm clicking on windows that are unusable.

I can't even exit or minimize Sourcetree by clicking, I can only use the keyboard shortcuts to look around, and I can't see a hidden window or anything. To continue to use Sourcetree, I need to Force Quit and re-launch the app.

This makes Sourcetree 100% unusable to me, so I reverted to 2.1 and it works fine.

The attachment shows the 'merge' I'm referring to (in Sourcetree 2.1 - same thing in 2.2 though)