bug#40532: 28.0.50; eww/shr: Anchor link does not work

["Basil L. Contovounesios" <contovob@tcd.ie>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: "Basil L. Contovounesios" <contovob@tcd.ie>
>> Date: Wed, 22 Apr 2020 12:38:17 +0100
>> Cc: 40532@debbugs.gnu.org
>> 
>> Arnaud Fontaine <arnau@mini-dweeb.org> writes:
>> 
>> > I have  investigated a little  and it  seems to be  because of a  bug in
>> > eww-follow-link (bound to RET key).
>> 
>> Indeed; I think it's a regression in Emacs 27 caused by the following
>> fix for bug#28441:
>> 
>> Make #anchors work again in eww
>> fa41693799 2018-04-13 14:55:55 +0200
>> https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=fa416937997a113d84ab4e4910d730ce5d77613d
>
> Then how come I cannot reproduce it in today's master?

0. emacs -Q
1. M-x eww RET https://en.wikipedia.org/wiki/Emacs RET
2. C-s Cul RET (place point on ToC entry "3 Culture")
3. RET         (eww-follow-link)

Expected: Point jumps to heading "Culture[edit]" at pos 21205 on L324
Observed: Point jumps to point-min
Emacs version details follow my signature.
What do you see on your end?

>> Yes, we wouldn't want eww-follow-link to do that, and in fact it already
>> contains logic to avoid reloading the whole page, modulo the
>> aforementioned regression in Emacs 27.
>> 
>> Lars, Eli, how's the following fix for emacs-27?
>
> I need first to understand what is going on, and in particular why it
> doesn't happen to me.

What's going on: shr.el currently requires its users to set
shr-target-id before rendering a DOM, so that it knows where to place
the eponymous text property.

The only user of this text property in the Emacs sources is
eww-display-html, which leaves point where the text property was placed
after shr is done rendering the DOM.

Prior to the patch for bug#28441, eww-display-html would always set
shr-target-id to the current url-target (or nil) prior to invoking shr.

The patch for bug#28441 removed the setting of shr-target-id from
eww-display-html and moved it to eww-render, so that it's set before any
URL redirections strip the original url-target away.

Since eww-follow-link calls eww-display-html directly, rather than going
through eww-render (which is a url-retrieve callback), shr-target-id is
no longer set appropriately when eww-follow-link is invoked on a URL
with a #target.  So shr.el doesn't know to insert the corresponding text
property, and eww-display-html leaves point at point-min rather than the
correct #target.

-- 
Basil

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
 of 2020-04-22 built on thunk
Repository revision: ab214143bbc633bcbe1ae146647c2fdc882122f0
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Debian GNU/Linux bullseye/sid

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Contacting host: en.wikipedia.org:443
uncompressing publicsuffix.txt.gz...done
Mark saved where search started

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O2 -march=native' --config-cache
 --prefix=/home/blc/.local --with-x-toolkit=lucid
 --with-file-notification=yes --with-x'

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LIBSYSTEMD JSON PDUMPER LCMS2 GMP

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  locale-coding-system: utf-8-unix

Major mode: eww

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Load-path shadows:
None found.

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Memory information:
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