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Re: Should invisible imply intangible?

From: Richard Stallman
Subject: Re: Should invisible imply intangible?
Date: Sun, 17 Mar 2002 03:06:02 -0700 (MST)

    I was specifically referring to the case where both properties come
    from the same overlay.  I don't know what should (or does) happen
    when the two properties come from different sources.

The simplest and cleanest thing to do is make the definition
of each property independent of how it gets applied.
We have to be very hesitant about making the meaning of a property
different for overlays vs text properties.

      Now in the case of overlays,
    those are not connected with single characters, but with buffer

They are connected with a range of buffer positions, which covers
a run of characters.

      If we are at a boundary where
    invisibility starts, the before-string is to be displayed before the
    overlay, before invisibility starts.

That rule could be coherent both for text properties and for overlays.
Another coherent alternative would be to say that when a character is
invisible its before-string and after-string do not appear.  I am not
sure which is better--perhaps whichever one is easier to implement.
But I think text properties and overlays should do the same thing.

    Now the question remains what to do with the display property itself.
    In case the overlay lies completely contained within an invisible area
    (as determined by text properties or another overlay with invisible
    property), it should not display.  That much is obvious.  But if start
    and/or end lie on the edge, what should we do?  If we really are
    pedantic, the behavior will depend on the stickiness of the
    invisibility borders.

The display property has nothing particularly to do with stickiness.
Whether it specifies an image or something else, in all cases it
modifies the appearance for of text.  Its definition should be fixed
and static, and not affected by stickiness.  If the text is invisible,
the display property should be ignored.

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