bug#56682: locked narrowing

[Juri Linkov]

To be able to fix bug#56815 I started to test the locked narrowing,
and everything works smoothly with:

  (setq treesit-max-buffer-size 1200000000)
  (push '(js-json-mode . json-ts-mode) major-mode-remap-alist)
  (add-hook 'find-file-hook 'toggle-truncate-lines)

Navigating 1 GB hugedictionary.json exhibits no delays
(the recipe uses treesit to get the correct highlighting).

However, 2 problems encountered:

1. after 'M-g TAB' (move-to-column) to 214748364 or more,
   the display is not updated anymore: moving point to the left
   from this position shows the cursor, moving point to the right
   has no visible effect.  Is it a hard limit in the display engine?
   Its hex value is #xccccccc.

2. after starting Isearch at a large column number,
   Emacs hangs up indefinitely, e.g. with
   'M-g TAB 10000000 RET C-s' then even C-g doesn't get out.
   Debugging shows that the problem is in 'isearch-update'
   where the call to 'pos-visible-in-window-group-p' doesn't return.
   When this call is removed, the search is instantaneous.
   (Optimizing lazy-highlight is a separate problem in bug#56815.)

PS: it seems these problems are not related to the locked narrowing,
rather the locked narrowing helped to expose them, so maybe they
should be reported in a new separate bug report?