As per summary, after maximizing the SourceTree window, moving the mouse pointer to the bottom of the screen does not reveal the (auto-hidden) Windows taskbar as it is meant to.

I usually hit the Windows key to get it to re-appear. I have two monitors setup - clicking on a window on the 2nd monitor also allows the taskbar to reappear when the mouse pointer is moved to the bottom of the screen.

I have not extended the taskbar onto the secondary monitor.

I have the same issue on a single monitor setup though.