bug#22250: 25.0.50; Eww fails to break RTL paragraph

[Benjamin Riefenstahl <b.riefenstahl@turtle-trading.net>]

Eli Zaretskii writes:
>> At this point every problem in my application and test cases that I
>> still see involves diacritics.
>
> I fixed that case now, please try the latest emacs-25 branch.  It was
> a very subtle problem with shr-vertical-motion (which actually
> triggered a bug in vertical-motion).

That works.  Thanks, that's cool. 


> I'll need a clear test case to look into this.

Try the attached patch.  It reverts parts of Lars' fix and adds a debug
message to shr-vertical-motion.

For a base-line test, execute

  ./emacs -Q -nw --eval '(eww "https://odoacer.turtle-trading.net/abc-abc-abc-abc-abc-abc-abc-abc-abc-abc-abc-abc-abc-abc-test.html")'

Once the page is loaded, press "G RET".  This second load reproduces the
problem for me.  I get this protocol in the message buffer:

  Contacting host: odoacer.turtle-trading.net:443
  bpd: right-to-left pt: 1 hscroll: 0
  bpd: right-to-left pt: 97 hscroll: 0
  bpd: right-to-left pt: 193 hscroll: 0
  bpd: right-to-left pt: 289 hscroll: 0
  Contacting host: odoacer.turtle-trading.net:443
  bpd: right-to-left pt: 1 hscroll: 57
  bpd: right-to-left pt: 153 hscroll: 57
  bpd: right-to-left pt: 305 hscroll: 57

The first run is as I expected.  The second run has point at 1 and
hscroll at 57 (this is in a terminal, that's why the actual number is
different from before).  According to my logic that should not be
possible.  When the point is at 1, then hscroll should be 0 otherwise
point would not be visible.  Unless some intermediate state is
permissible.  But than shr could not rely on hscroll and therefore not
on vertical-motion.

Now as a second experiment, remove the ";" from bidi-paragraph-direction
in shr-insert-document.  Repeat the test.  Now the result should look
correct.  Somehow bidi-paragraph-direction does make a difference.