If you have your Windows taskbar  set to hide and on the left side of your screen and maximize SourceTree, the taskbar is covered and shoving your mouse to the left side of the screen doesn't activate the taskbar. You have to Alt+Tab to another program or use the Win key to show the taskbar.

Steps to Reproduce

1. Set your Windows task bar to be located on the left side of the screen and to automatically hide.
2. Shove your mouse pointer to the left side of the screen and see the taskbar appear.
3. Open SourceTree and maximize it.
4. Shove your mouse onto the left side of your screen and see the taskbar doesn't appear.