bug#43105: (window-body-height) Reporting Too Large of Value

[Eli Zaretskii <eliz@gnu.org>]

> From: William Carroll <wpcarro@gmail.com>
> Date: Sat, 29 Aug 2020 18:10:17 +0100
> 
> I believe `(window-body-height)` does not account for non-zero `line-spacing` amounts. This causes
> `(window-body-height)` in graphical Emacs to report values larger than the number of lines of text that can
> render on the screen.

window-body-height reports the height of the window's body in units of
canonical character height:

  If PIXELWISE is nil, return the largest integer smaller than WINDOW’s
  pixel height divided by the character height of WINDOW’s frame.
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Thus, window-body-height is by design insensitive to factors like
non-default fonts, line-spacing, etc.

> This affects programs like vterm.el and others that rely on `(window-body-height)`. In my particular case,
> when I ran `man` and `less` from vterm.el, it rendered things above the top "fold" of the screen.

Then the bug is in vterm.el: it should use other APIs to get the
dimensions in terms of actual number of lines in the window.  The
function window-body-height works as intended.

Thanks.