From: Gregory Heytings <[email-masked]>
Date: Sun, 20 Sep 2020 01:00:54 +0000

A short note, which could perhaps be useful: this behavior does not only 
happen when the overlay text is too wide for the minibuffer width (in 
which case it perhaps could be argued that the display engine tries to do 
something appropriate).

For example, with (setq icomplete-separator "\n"), after M-x a the 
contents of the minibuffer is displayed as follows:

{rp
lign
propos
sm-mode
<... other candidates, one on each line>

After pressing C-b (or <left>), the prompt becomes visible, and the 
minibuffer is displayed as follows:

M-x a{rp
lign
propos
sm-mode
<... other candidates, one one each line>

(Note that the display position of the candidates is not modified, it is 
still aligned to the left border of the window, except for the first one.)

Pressing C-f (or <right>) returns to the initial display, the prompt 
becomes invisible again.