bug#64724: 30.0.50; Inconsistency between manual, comments in the code, and implementation of point adjustment

[Ihor Radchenko <yantar92@posteo.net>]

Hi,

This is a followup of https://orgmode.org/list/87ilag1pth.fsf@localhost

We recently ran into some subtleties of point adjustment near invisible
regions with sticky 'invisible property.

The actual code in `set_point_both' makes sure to move point across
invisible sticky region if point happens to end up right before front
sticky invisible or right after rear sticky invisible region.

However, 22.6 Adjusting Point After Commands does not mention this
scenario.

Further, `set_point_both' contains the following comments:

	  /* If the preceding character is both intangible and invisible,
	     and the invisible property is `rear-sticky', perturb it so
	     that the search starts one character earlier -- this ensures
	     that point can never move to the end of an invisible/
	     intangible/rear-sticky region.  */
	  charpos = adjust_for_invis_intang (charpos, -1, -1, 1);
          <...>
      	  /* If the following character is both intangible and invisible,
	     and the invisible property is `front-sticky', perturb it so
	     that the search starts one character later -- this ensures
	     that point can never move to the beginning of an
	     invisible/intangible/front-sticky region.  */
	  charpos = adjust_for_invis_intang (charpos, 0, 1, 1);

However, 'intanglible text property is, in fact, not necessary to
trigger the adjustment.

May you (1) document this behaviour in the manual; (2) clarify what's up
with intanglible.

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.17.8) of 2023-07-18 built on localhost
Repository revision: aba3d13ea8d712bca1b3f23ac7db7e38c2165b3c
Repository branch: feature/named-lambdas
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Gentoo Linux

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
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