bug#30432: 26.0.91; freeze when text wrap-prefix exceeds window text width

[Eli Zaretskii <eliz@gnu.org>]

> From: Paul Rankin <hello@paulwrankin.com>
> Date: Mon, 12 Feb 2018 16:49:39 +1000
> 
> 1. emacs -Q
> 2. make sure window is 80 columns wide
> 3. insert a block of text that exceeds the window text width and causes wrapping
> 4. set region over this text
> 5. (put-text-property (region-beginning) (region-end) 'wrap-prefix '(space :align-to 81))
> 6. move point to or past this text
> 
> Expected result:
> 
> I'm not sure, but I guess window should scroll horizontally to display point.

It can't.  (And anyway, horizontal scrolling only happens when lines
are truncated.)

What the above wrap-prefix does is ask for 81 columns of whitespace
each time we need to continue a line on the next screen line, before
the text of that continuation line.  But 81 columns is more than the
window can show, so the wrap prefix itself causes another continuation
line to appear, which then needs another wrap prefix, etc. etc., ad
nauseam.

IOW, asking for a 81-column wrap-prefix in a 80-column wide window
makes no sense, and leaves redisplay no way of showing point -- or
even showing more than one screen line of text.

> Actual results:
> 
> Emacs freezes and needs to be killed.

I don't think I see a freeze here.  What I see is that the cursor
disappears, and Emacs repeatedly enters redisplay (because point is
not visible) -- which is why you might think Emacs freezes.  But if
you type C-b enough times, or even just type M-<, cursor will
eventually reappear, the "freezing" will stop, and Emacs will start
behaving normally again.  Although you sometimes need to wait for that
to happen for a few dozen seconds.

Is that what you see?  Or do you see something different?  (For the
record, I inserted a 90-character line into *scratch*, then marked it,
and typed the put-text-property form.)

> In Emacs versions prior to 26, this use to just wrap the transition text, but something has changed with 26 to cause the freezing.

What do you mean by "just wrap the transition text"?  Can you tell
what you see in previous versions with the recipe above?

What I see in previous versions of Emacs is something similar to Emacs
26, except that the cursor instead of disappearing jumps to the end of
the first screen line (which is a clearly bogus location), and stays
there until you type C-b enough times to get point into the visible
portion of the text line inserted at the beginning of the recipe.  Do
you see something different?

Thanks.