bug#27103: Should show-trailing-whitespace highlight specified spaces?

[Clément Pit--Claudel]

Hi emacs-devel,

I'm trying to display "/// " comment markers as a thin vertical line. At first, 
I went for something like the following font-lock rule:

  '(face my-vertical-line-face display (space :width 0.5))

This worked fine, until I decided to add a fringe icon, too:

  '(face my-vertical-line-face display [(space :width 0.5) (left-margin 
left-arrow)])

Now (with show-trailing-whitespace set to t) Emacs highlight lines that just 
contain "///" in bright red.  Here's a quick way to reproduce this:

  (dotimes (_ 25) (insert (propertize "///" 'display '((space :width 10) 
(left-fringe left-arrow))) "\n"))

Contrast with this, which doesn't show trailing-whitespace highlights:

  (dotimes (_ 25) (insert (propertize "///" 'display '(space :width 10)) "\n"))

Am I doing something wrong?  Or is this a bug, and if so which behavior is the 
correct one? Is there a way to disable the trailing whitespace highlighting 
here?

The best I've managed is to put the left-fringe property on the first "/" 
character, and the display property on the next two, but this interacts in 
annoying ways with point motion.  I added cursor-intangible properties to the 
mix too, but no luck (they interacted badly with insertion/deletion)

Thanks,
Clément.

signature.asc
Description: OpenPGP digital signature