After using SourceTree for multiple hours, when selecting a single commit, the commit window looks like this:

Displaying all changes between df1a55194519084fadd437c1bc1a1cc06fba5d85 and 082b71fcffa85648a215764d56f8282cc34c0965

But visually, only a single commit is selected.  ctrl-selecting multiple commits doesn't even visually work at this point, and I am unable to "reset" the UI except by restarting the app.

Unfortunately, I'm unaware of any particular steps that can be taken to reproduce this issue, but it consistently happens on my PC.  

Note:  I'm aware of one thing I'm certain is not involved in reproducing this issue, which is ctrl-selecting multiple commits.  This is not a feature I normally use, and in fact only became aware it existed because of this bug.

I've discovered a workaround, which is switching to another repository and back.