Extra vertical gap above the modeline when using tmux #663

Ghostel is amazing! But I found what seems to be an incorrect vertical alignment when running tmux.

In a raw Bash shell, there is no gap between the modeline and the prompt. See in the example below how the number 4 is truncated so that the prompt lands perfectly above the modeline. I think this is the proper behavior.

But if I run a tmux instance, a gap appears between the tmux status bar and the modeline. See how the 6 is fully shown. The first line (6, here) is never truncated, because of the spurious gap between the tmux status bar and the modeline. I assume this is not expected, and that the proper behavior would be to remove the gap between the tmux status bar and the modeline, and show a truncated 5.

The analysis of an AI is the following:

With these measurements:

(window-screen-lines) => 73.71428571428571

Ghostel normally rounds this down to 73 terminal rows. With a regular shell, ghostel--pixel-anchor can use the scrollback and applies a small vertical scroll, so the bottom is aligned correctly.

With tmux, Ghostel is on the alternate screen, which has no Ghostel scrollback above the visible terminal. ghostel--pixel-anchor then returns:

(1 . 0)

If Ghostel uses 74 rows (ceiling instead of floor), it returns:

(1 . 4)

and the display is aligned as expected: the first line is slightly truncated and the tmux status bar is flush with the modeline.

If that helps, here is a fix I applied in my Emacs configuration, where ceiling makes all the difference:

(defun my/ghostel-window-size (process windows)
  (let ((size (window-adjust-process-window-size-smallest process windows)))
    (if (and windows
             (buffer-live-p (process-buffer process))
             (with-current-buffer (process-buffer process)
               (derived-mode-p 'ghostel-mode)))
        (cons
         (car size)
         (max
          1
          (ceiling
           (apply
            #'min
            (mapcar
             (lambda (window)
               (with-selected-window window
                 (window-screen-lines)))
             windows)))))
      size)))

(setq-default window-adjust-process-window-size-function
              #'my/ghostel-window-size)

Thanks for the excellent work on this very useful extension!