Eli Zaretskii <eliz@gnu.org> writes:

>
> There's already such a calculation in window_scroll_pixel_based:
>
>       /* See if point is on a partially visible line at the end.  */
>       if (it.what == IT_EOB)
> 	partial_p = it.current_y + it.ascent + it.descent > it.last_visible_y;
>       else
> 	{
> 	  move_it_by_lines (&it, 1);
> 	  partial_p = it.current_y > it.last_visible_y;
> 	}
>
> (This is preceded by moving the iterator to the point's screen line or
> to EOB, whichever comes first.)  The value it.current_y is the Y
> coordinate of the top edge of a glyph row (the value is zero for the
> first screen line), so if it.last_visible_y is farther away from that
> than the height of the glyph row, that glyph row is fully visible;
> otherwise it isn't.

I'm not entirely clear why there is a branch in that code.  I came up
with this; it works, though I'm not sure if it's the best way: