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bug#62445: 13.1.10; bug in preview-region applied to repeated math envir

From: Paul Nelson
Subject: bug#62445: 13.1.10; bug in preview-region applied to repeated math environments
Date: Sat, 25 Mar 2023 17:14:48 +0100

Emacs  : GNU Emacs 29.0.50 (build 3, x86_64-apple-darwin21.6.0, NS appkit-2113.60 Version 12.6 (Build 21G115))
 of 2022-11-06
Package: 13.1.10

Take the following document:
#+begin_src latex
 $x$ $x$

Run ~preview-region~ on the region consisting of the first $x$.  This works as expected.

Next, run ~preview-region~ on the region consisting of just the second $x$.  This does not preview the second $x$.  Instead, it refreshes the overlay on the first $x$.

The relevant output:

./_region_.tex:5: Preview: Snippet 1 started.
l.5 $
Preview: Tightpage -32891 -32891 32891 32891
./_region_.tex:5: Preview: Snippet 1 ended.(282168+0x374556).
l.5 $x$

The issue here is that there's not enough context for ~preview-parse-messages~ to determine which $x$ to overlay.

One workaround would be to apply ~preview-region~ to a region containing both $x$'s (or to use ~preview-buffer~, etc).  This workaround wasn't ideal for my use-case (see parenthetical comment at the bottom).

I think a fix would be to store the beginning of the region being previewed in a buffer-local variable and, if needed, bump the point when searching for where to place the overlay.  Details:

- Add the following line somewhere in preview.el:
  #+begin_src elisp
  (defvar-local preview-region--begin nil "Start of region being processed.")

- Add the following line to ~preview-region~, just before the invocation of ~preview-generate-preview~:
  #+begin_src elisp
  (setq-local preview-region--begin begin)

- Add this to ~preview-parse-messages~, just before the second ~cond~ block:
  #+begin_src elisp
  (when (< (point) preview-region--begin)
    (goto-char preview-region--begin))

I've made these changes in my local version and they have worked for me.

(This bug was an issue for me because I had set up a timer that searches the visible portion of a TeX buffer for unrendered math environments and runs ~preview-region~ on contiguous unpreviewed blocks.  This led to many situations where  ~preview-region~ was called on math regions, such as the second $x$ in the above example, that had already been previewed in the current line.  My original workaround was to enlarge the region sent to ~preview-region~ to contain any repeated math regions that appear earlier in a given line.  This worked, but often resulted in many fragments getting unnecessarily refreshed.)

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