Steps to reproduce:

1. Use SourceTree for some time - open & close repository details, commit, merge etc.
2. Try closing some repository detail window. Does not matter whether using Cmd+W or clicking the top-left red close button.
3. The window takes several seconds to close, appears stuck in the meantime (not animating). The closing seems to get worse the longer you use SourceTree.

After restarting SourceTree, the problem disappears - and then starts appearing again after a few actions with it.

An important observation is that this problem started with the version 2.6.1 - it was not present in any of the previous versions as far as I can tell (certainly not in 2.5.x - would have noticed).