🚀 go-pugleaf

Emacs version is 18.57.

I came across this problem while using Barry Warsaw's Supercite 2.2.  I
filled a cited paragraph with justification turned on and it was
justified to the wrong column.  After a bit of digging around I found
the fault was in Emacs' fill.el package.

Reproduce as follows:-

Switch to a buffer in fundamental mode, set a fill column, turn on
auto-fill (for convenience), type in a few letters (eg "xxx> ") and make
this the fill prefix with C-x .

Now type in a few lines of gibberish...

xxx> gibberish gibberish gibberish gibberish gibberish gibberish
xxx> gibberish gibberish gibberish gibberish gibberish gibberish
xxx> gibberish gibberish gibberish gibberish gibberish gibberish
xxx> gibberish gibberish gibberish gibberish gibberish gibberish

Now position point within the paragraph and do M-1 M-q to fill and
justify...

xxx> gibberish   gibberish  gibberish    gibberish     gibberish    gibberish
xxx> gibberish     gibberish  gibberish    gibberish  gibberish     gibberish
xxx> gibberish  gibberish     gibberish  gibberish    gibberish     gibberish
xxx> gibberish gibberish gibberish gibberish gibberish gibberish

I did the above with fill-column set to 72.  It's been justified to
xxx> column 77, which is (+ fill-column (length fill-prefix)).


The fault is in `justify-current-line' in fill.el.  This does a
`narrow-to-region' on the text from the end of the fill prefix to the
end of the line, then pads that out to fill-column.  Hence the extra
characters.

The fix is to change the LET line from:-

    (let (ncols beg)

to:-

    (let ((fill-column (- fill-column (length fill-prefix)))
	  ncols beg)
      (if (< fill-column 1)
          (error "Fill prefix is too long for fill column."))

This compensates for the extra characters on the line.

Kevin