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bug#33664: 26.1; Document vars and functions in `cursor-sensor.el'

From: Stefan Monnier
Subject: bug#33664: 26.1; Document vars and functions in `cursor-sensor.el'
Date: Sun, 09 Dec 2018 14:35:42 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

> Specifically, I'd like to know some behavior
> differences - examples of when `intangible' text

With `intangible` you can have a situation where a chunk of code does

    (goto-char POS)

and it misbehaves because BLAH BLAH BLAH expects to run with point at POS,
but some other package added an `intangible` property so the goto-char
actually ended up putting point somewhere else.

IOW `intangible` breaks all kinds of invariants like

    (point) == (progn (forward-char N) (forward-char -N) (point))
    (+ N (point)) == (progn (forward-char N) (point))

[ Admittedly these invariants aren't true when you bump into EOB but
  a lot of code is prepared to deal with odd cases at EOB but is not
  prepared for such surprises happening virtually anywhere.  ]


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