I suppose there are two ways to address this:
1) Allow alignment at the top left, but make sure there is no fractional
line at the bottom. Instead, allow there to be a fractional line of
white space at the bottom.
2) Alignment at the bottom left, which means that the top of the
terminal window is where there will be a fractional line.
I kind of prefer the second behavior for the reasons that the top of the
window isn't where the user's eyes are drawn when typing and also a
fractional line at the top does give the impression that the text is
scrolling upward and there might be additional lines scrolled past the
top of the window.