Can't re-dock a window/tab

I double-click a file from the “Commit” section to see a diff and “undocked” it to put on a second screen. But now I can't redock it to the main window and every other diff I open, it does in a new window.

I tried : 
 - Invalidate Caches
 - Restore to the default layouts
 - Drag n Drop the window into the main window (does nothing)