bug#50178: 28.0.50; Size of echo area does not account for line-spacing

[Gregory Heytings]

> On true pixel-oriented systems there are APIs for querying the display 
> engine about several metrics. Then you can place the text at certain 
> pixel coordinates. Emacs, however, is a Frankenstein system, that uses 
> pixels (on graphic frames) but the text positioning depends on previous 
> text, i.e. for vertical positioning it is a line-based, not pixel-based, 
> system. Therefore, when you just need to output some lines, you must 
> deal with pixels, translate back to lines and, to add insult to injury, 
> resort to post-facto information.
>
> As useful as it would be an API that returns how many lines fit on a 
> given window. Or, on this case, max-mini-window-height being a true 
> indication of the capacity of the mini window on terms of the current 
> display settings, which is what the users want 99.9% of the time.

Try emacs -Q, C-h h to see why it is impossible to have "an API that 
returns how many lines fit on a given window".