WindowFloating is an option for Notebook which specifies whether the window for the notebook should float on top of other windows when it is displayed on the screen.
WindowFloating->True is often used for palettes.
If there are several floating windows the most recently selected one goes on top.
See The Mathematica Book: Section 2.10.2 and Section 2.10.13.
See also: WindowClickSelect, Visible.
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