bug#43519: 28.0.50; Overlay at end of minibuf hides minibuf's real content

[Gregory Heytings]

> IIUC the problem only shows up because of the auto-resizing of the 
> minibuffer window, right?

Yes.

> Indeed if I replace
>
>    (setq max-mini-window-height 1)
> with
>    (setq resize-mini-windows nil)
>
> the problem doesn't appear, even though resizing is in fact disabled in 
> both cases.

Yes, but setting these two variables have a very different effect. 
Setting resize-mini-windows to nil means that resize_mini_window() will 
return immediately after setting the start of display to its default value 
(namely BEGV).  Setting max-mini-window-height to 1 does not have that 
effect, resize_mini_window() computes the height that would be necessary 
to display the content of the minibuffer, compares it to the height that 
is actually available, and uses that height to calculate the start of 
display, by moving backward from the height that would be necessary (which 
does nothing in this case, given that height == unit).