Thanks for that last bit. That sort of jives with a bug that affects some linux systems RE the F&R widget. Although the overall Window size doesn't seem to be affected on Linux, when the F&R widget is open when exiting Sigil, the central dock area (Code View/Book View) will expand (when sigi! is next opened) and "smoosh" the left and right dockwidget areas down to almost nothing. The same workaround prevents it.
So in short: it's not just you!

But this is the first I've heard of something similar to it happening on a Mac. This one has been on my radar for a while now.
P.S. Do you typically have the Sigil Window maximized when you use it? Whether or not the Sigil window is maximized when closing also seems to have some sort of bearing on this bug (at least on the affected Linux systems).