This first happened after upgrading from 1.10.xx to 2.0.19.1. (Windows 10 Pro - Anniversary Update)

Frequency: This happens every time. My co-workers on 2.0.19.1 also experience this issue.

To reproduce:
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.