RE: How can I remove effects of text properties in 'before-string

[Drew Adams <drew.adams@oracle.com>]

> > (set-window-margins (get-buffer-window) 20 0)
> > (overlay-put (make-overlay (point) (point)) 'before-string
> > (propertize "." 'display `((margin left-margin) ,"Hi there!")))
> > (hl-line-mode)
> >
> > The string "Hi there!" is also highlighted. How can I disable
> > that?
> 
> Give the string a distinct face.

Why doesn't it use the `default' face?  If no `face'
property is given to the string, why would the effective
face be something different from `default'?

Where in the doc do we say that this should use a face
that is used in an overlay placed on some buffer positions?

Is this behavior documented?  I couldn't find anything
about it, but it might well be there somewhere.  I looked
in the doc about overlays and the doc about the margin,
but perhaps it is there and I overlooked or misread it.

Or is this perhaps a bug?  For what reason should the
properties of an overlay on some parts of a buffer
affect the appearance of the margin?