WindowMovable is an option for Notebook which specifies whether to allow the window for the notebook to be moved around interactively on the screen.
WindowMovable affects only interactive operations in the front end.
Even with WindowMovable->False, the WindowMargins option can still be reset from the kernel or option inspector menu.
See The Mathematica Book: Section 2.10.13.
See also: WindowMargins, Selectable.
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