I have re-read the page https://www.eiffel.org/doc/eiffelstudio/Customizing_the_tools_layout_and_toolbars at section on "Floating a pane", and crosschecked my Preferences section after that.

My conclusion as of this writing:

Preferences settings "interface.development_window.undocked_window_lower_than_main_window" had been set my me marked / true some time far back. As such it did allow the floating window go out of sight. 

Request: Please review the functionality of "going out of sight" (aka losing focus?). 

Reason: Just moving an opened floating window into a better location by click-and-draging its title area will make it disappear at button-up, leaving me unable to do intended work. It's very vulnerable to disappear when one accidentially clicks outside of its window area. Also there seems a problem to get it back. It seems that the View>Tools>xyz (or keyboard shortcut) does not trigger it to re-appear once opened. 

Note: I don't know if this a general problem or one of the Windows implementation under Win 8.1(Pro).

Note: the nasty workaround to get a floating window back was mentioned in the initial post: use toolbar reconfiguration button will pull up all hidden floating tool windows.