I happen to use the screen tiling utility, “Mosaic” http://www.lightpillar.com/mosaic.html .Today I noticed something peculiar:
If I had my usual vast number of buttons stuck in my Scrivener toolbar, I could not convince Mosaic to squeeze my Scriv window down so it would fit on my iPad when used as a second screen for my Mac–even though I could manually shrink it to fit. If, OTOH, I removed a lot of toolbar buttons, Mosaic would shrink my window to fit quite handily.
Intrigued, I went on to try some other window tile shapes in Mosaic. In all cases, if I had a lot of toolbar buttons, Mosaic would not shrink the Scrivener window as far as I could manually. If I removed toolbar buttons, Mosaic worked fine. Interestingly, if I hid either the Binder or the Inspector, Mosaic behaved as expected, with the little >> menu at the end of the toolbar displayed. But if I displayed either, Mosaic would insist on displaying the entire toolbar without the extras >> menu at the end.
It appears to me that Scrivener passes a number to Mac OS representing its minimum window dimensions, which Mosaic then uses. This number does NOT take into account that the toolbar can shrink smaller than required to display all buttons (with extra buttons showing up in the little >> menu at the end) if either sidebar is displayed.
I suggest that this is a bug. I am open, however, to alternative explanations. 
