If the SourceTree window is maximised on one monitor, it cannot be dragged to another monitor - this is inconsistent with every other application installed on my machine.
You either have to restore (i.e. un-maximise) the window first, move it and re-maximise it, or use the Windows 10 hotkeys: Win+right/left arrow.
- duplicates
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                    SRCTREEWIN-6983 Sourcetree window drag behaviour is wrong when maximized -         
- Closed
 
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                    SRCTREEWIN-7133 Unable to drag full screen window -         
- Closed
 
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- is duplicated by
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                    SRCTREEWIN-7317 Moving sourcetree window when full screen -         
- Closed
 
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                    SRCTREEWIN-7382 Can't move the window via title/menu bar drag when maximized. -         
- Closed
 
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                    SRCTREEWIN-7220 Cannot reposition window when in full screen mode -         
- Closed
 
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                    SRCTREEWIN-7367 Window maximized issue -         
- Gathering Impact
 
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