Subject: 24.4.50; Feature Request -- calculate new `window-start` & `window-end` before visual redisplay

From: Keith David Bershatsky <[email-masked]>
Date: Tue, 03 Jun 2014 10:48:54 -0700

At the present time, I do not believe that Emacs has the ability to calculate a new `window-start` and a new `window-end` without a redisplay occurring, because it needs to take into account things like the new cursor position, `visual-line-mode`, `linenum-mode`, etc.

Functions assigned to the `post-command-hook` use the `window-start` and `window-end` positions that were previously visible before a new redisplay occurs.

Example:

* Long buffer of text that exceeds the `window-height`.

* Jump interactively from `beginning-of-buffer` to `ending-of-buffer`, or visa-versa.

* In the `post-command-hook`, have the following two elements:

  ** Remove overlays xyz between the *old* `window-start` and *old* `window-end`.

  ** Add overlays xyz between the *new* `window-start` and *new* `window-end`.

The final component above (i.e., ** Add overlays xyz between the *new* . . .) cannot be done correctly without a forced `redisplay` occurring.  The result of a forced redisplay is that the user will see a momentary visual representation of the buffer *without* overlays xyz.  The forced redisplay permits Emacs to correctly calculate the *new* `window-start` and *new* `window-end`.

In a stackoverflow thread regarding this issue, @Stefan suggested that I take a look at the `jit-lock-register`.  After combing through the code in `jit-lock.el` and trying some experiments, I believe that `jit-lock.el` is more suited to a situation when buffer *modification* occurs -- it is also suited for situations where one may wish to incrementially act upon an entire buffer in small chunks at a time to increase efficiency of avaialble resources.

http://stackoverflow.com/questions/23923371/emacs-calculating-new-window-start-end-without-redisplay

I believe that the solution may be to have a `sneak-preview-redisplay` whereby the *new* `window-start` and *new* `window-end` could be correctly calculated without the visual buffer actually being redisplayed.

Thanks,

Keith

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In GNU Emacs 24.4.50.1 (x86_64-apple-darwin10.8.0, NS appkit-1038.36 Version 10.6.8 (Build 10K549))
 of 2014-06-01 on MP.local
Repository revision: 117215 [email-masked]
Windowing system distributor `Apple', version 10.3.1038
Configured using:
 `configure --with-ns'

Configured features:
ACL LIBXML2 ZLIB

Important settings:
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Major mode: Fundamental

Minor modes in effect:
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