Weird. I've never had a widget go true full-screen on me before. I'm always able to grab the top bar and move it left/right/up/down (even if it moves some parts off-screen).
If worse comes to worse, you could manually edit the sigil.ini preference file to reset all the widgets/toolbars to their defaults. Or if you don't have much in the way of customization, you could just delete the sigil.ini file completely and start over.
The sigil.ini file is found in the
Sigil Preferences folder.
If you don't want to delete it entirely, you could remove the three lines that begin with:
maximized=
geometry=
toolbars=
They can be found under the [mainwindow] section. The last two will be incredibly long bytearrays. More than likely you'll be able to delete everything from (and including) "maximized=" down to (but exluding) [preferences_dialog] .
Just make sure Sigil isn't open when you delete the ini file, or when you edit/save it, if that's what you decide.