I noticed one nit - after clearing history you can press Scroll Lock and scroll up through the cleared history buffer, while we probably want to reset that as well. However, I'd suggest that you commit this now and we can address that in a followup. I also did not check how sc(4) behaves with respect to this.
From what I can tell, it looks like sc(4) just clears the entire buffer using sc_vtb_clear() without making additional adjustments. But I agree that it doesn't seem useful to be able to scroll back through hundreds of lines of nothing. It looks like resetting the scrollback state should be pretty simple; I'll post a follow-on change to do that.