bug#29789: 25.1; Emacs blocks user input when using visual-fill-column in wide terminals

[Eli Zaretskii <eliz@gnu.org>]

> From: Luis Gerhorst <privat@luisgerhorst.de>
> Date: Wed, 20 Dec 2017 20:28:27 +0100
> 
> 
> 1. Run Emacs in a wide terminal (about 150 characters, 100 isn't
> enough): emacs -nw -Q
> 
> 2. Load a arbitrary theme: M-x load-theme adwaita
> 
> 3. Load visual-fill-column. E.g. by copying
> https://raw.githubusercontent.com/joostkremers/visual-fill-column/c0e5ec8f88a7598f7afdee565978410bee0de2a0/visual-fill-column.el
> into the scratch buffer and typing M-x eval-buffer.
> 
> There is nothing suspicious about the code, regular lisp code should not
> be able to block Emacs (I believe). It does not seem to contain any loops that block the main thread. The maintainer knows about the issue but say's it may be an issue with low-level code. In GUI Emacs it works.
> 
> 4. Open a buffer and enable visual-fill-column-mode: M-x
> visual-fill-column-mode
> 
> 5. Your Emacs is now blocked.

I tried to reproduce this on GNU/Linux and on MS-Windows, but failed
both times, both with Emacs 25.1 and the current emacs-26 release
branch.

What I see is that after enabling the mode, Emacs becomes somewhat
sluggish in cursor motion (hardly surprising, given what the mode
does), but it certainly doesn't hang.

I wonder what's different on your system, or on that of those who
confirmed the problem.