Does adjusting the Left/Right editor margin settings, in the Appearance: Main Editor: Options tab of File/Options... help? I have seen overlap of interface elements in a few cases. I don’t think I’ve seen it with scrollbars, but the line numbering feature (View/Text Editing/Show Line Numbers) can very easily overlap the left edge of text.
Technically I would call this a bug, where adding extra margin padding seems to be the current workaround. Now that I’ve tested this a bit more, I do get the behaviour you noted with Scrivenings + Composition.