A user reported:

Every now and then moving the vertical splitter gets stuck. The way to make it unstuck is to make SourceTree not be fullscreen and resizing the window. Then you can move the splitter again to make the view showing the file changes bigger.

I haven't been able to find a consistent way to reproduce this. However, I switch between fullscreen and not fullscreen, and also between one of several clones of various source code repositories.

Sometimes it gets stuck when intending to move the line to the left.