Emacs 22.0.50: Problem with Longlines mode and left margin

[Alan Mackenzie]

Hi, Emacs!

In a fresh buffer, set up a non-null left margin, e.g. with M-: (setq
left-margin 3).  Enable Longlines Mode.

Start typing a line of text, starting with three spaces.  When the text
fills onto the next (soft) line, Longlines disregards the left margin,
and text accumulates from column 0, like this:

   Text starting from the margin overflows
into column 0

.  Fill the paragraph with M-q.  It now appears to respect the margin
setting:

   Text starting from the margin overflows
   into column 0

.  However, type any character (or delete any character) anywhere in this
paragraph, and it snaps back to the margin-violating version.

Longlines's filling and M-q are incompatible here.  I think that it is
Longlines's filling which is at fault.

-- 
Alan Mackenzie (Munich, Germany)