This first happened after upgrading from 1.10.xx to 18.104.22.168. (Windows 10 Pro - Anniversary Update)
Frequency: This happens every time. My co-workers on 22.214.171.124 also experience this issue.
1. Maximize SourceTree
2. Click (and hold) on the title bar and drag your mouse down
3. Sourcetree window doesn't flip back to window mode like it should
Note: This does work properly if you click and drag from the top 1 pixel row of the title bar. It should work when clicking on any part of the title bar.