bug#17678: 24.4.50; Feature Request -- calculate new `window-start` & `window-end` before visual redisplay

[Keith David Bershatsky <esq@lawlist.com>]

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 lekktu@gmail.com-20140601162519-4dacx2w0ak528z2r
Windowing system distributor `Apple', version 10.3.1038
Configured using:
 `configure --with-ns'

Configured features:
ACL LIBXML2 ZLIB

Important settings:
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  lawlist-scroll-bar-mode: t
  lawlist-delete-selection-mode: t
  lawlist-linum-mode: t
  lawlist-visual-line-mode: t
  vr-mode: t
  hr-mode: t
  flyspell-mode: t
  desktop-save-mode: t
  tabbar-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  transient-mark-mode: t

Recent input:
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <M-right> <M-right> <M-right> <M-right> <M-right> 
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<M-right> <M-right> <M-right> <M-right> <right> <right> 
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<right> <right> <right> <right> <right> <right> <right> 
<right> <right> <right> <right> <right> <right> <right> 
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<M-right> <M-right> <M-right> <M-right> <M-right> <M-right> 
<M-right> <s-up> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <up> <M-right> <M-right> <M-right> 
<M-right> <M-right> <M-right> <M-right> <M-right> <M-right> 
<M-right> <M-right> <M-right> <M-right> <M-right> <M-right> 
<M-right> <M-right> <M-right> <M-right> <M-right> <M-right> 
<M-right> <M-right> <M-right> <M-right> <M-right> <M-right> 
<M-right> <M-right> <M-right> <M-right> <M-right> <M-right> 
<M-right> <M-right> <M-right> <M-right> <M-right> <M-left> 
<M-left> <M-left> <M-left> <M-left> <M-left> <M-left> 
<M-left> <M-left> <M-left> <M-left> <M-left> <M-left> 
<M-left> <M-left> <M-left> <M-left> <M-left> <M-left> 
<M-left> <M-left> <M-left> <M-left> <M-left> <M-left> 
<M-left> <wheel-down> <double-wheel-down> <triple-wheel-down> 
<wheel-down> <double-wheel-down> <triple-wheel-down> 
<wheel-down> <double-wheel-down> <triple-wheel-down> 
<wheel-down> <double-wheel-down> <triple-wheel-down> 
<triple-wheel-down> <wheel-down> <double-wheel-down> 
<triple-wheel-down> <wheel-down> <double-wheel-down> 
<triple-wheel-down> <wheel-down> <double-wheel-down> 
<triple-wheel-down> <wheel-up> <double-wheel-up> <triple-wheel-up> 
<wheel-up> <wheel-up> <double-wheel-up> <triple-wheel-up> 
<help-echo> <down-mouse-1> <mouse-1> <wheel-up> <double-wheel-up> 
<triple-wheel-up> <triple-wheel-up> <wheel-up> <double-wheel-up> 
<triple-wheel-up> <down> <down> <down> <down> <down> 
<up> <up> <up> <up> <up> <s-up> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <s-right> <left> <left> 
<left> <left> <left> <left> <left> <left> <left> s-N 
s-w k <left> <left> <left> <left> <left> <left> <left> 
<left> <left> s-N s-w k <escape> x e m a c s - b u 
g <return> <backspace> <backspace> <backspace> <backspace> 
<backspace> <backspace> <backspace> <backspace> <backspace> 
r e p o r t - e m a c s - b u g <return>

Recent messages:
Saving...done
Mark set
[k]ill or [s]ave draft?
Loading msgdb for +/Users/HOME/.0.data/.0.emacs/.0.mail/msgdb/imap/mail.lawlist.com/lawlist/INBOX.Drafts/spool...done
Type C-c C-x C-z to exit MIME mode, and type C-c C-x ? to get help.
Saving...done
Mark set
[k]ill or [s]ave draft?
Loading msgdb for +/Users/HOME/.0.data/.0.emacs/.0.mail/msgdb/imap/mail.lawlist.com/lawlist/INBOX.Drafts/spool...done
*beep*

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/Users/HOME/.0.data/.0.emacs/.0.simi/pgg hides /Users/HOME/.0.data/.0.emacs/Emacs_06_01_2014.app/Contents/Resources/lisp/obsolete/pgg
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/Users/HOME/.0.data/.0.emacs/.0.simi/pgg-pgp hides /Users/HOME/.0.data/.0.emacs/Emacs_06_01_2014.app/Contents/Resources/lisp/obsolete/pgg-pgp
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/Users/HOME/.0.data/.0.emacs/.0.simi/pgg-gpg hides /Users/HOME/.0.data/.0.emacs/Emacs_06_01_2014.app/Contents/Resources/lisp/obsolete/pgg-gpg
/Users/HOME/.0.data/.0.emacs/.0.simi/pgg-def hides /Users/HOME/.0.data/.0.emacs/Emacs_06_01_2014.app/Contents/Resources/lisp/obsolete/pgg-def

Features:
(shadow emacsbug modb-legacy wl-mime mime-edit pgg-parse pccl pccl-20
signature mime-setup mail-mime-setup semi-setup mime-pgp pgg-def
mime-play filename mime-image modb-standard elmo-imap4 disp-table
bbdb-autoloads lawlist-wl wl-demo wl-draft eword-encode wl-template
sendmail elmo-net elmo-cache elmo-map elmo-dop wl-news wl-address
wl-thread wl-folder wl wl-e21 wl-spam wl-action wl-summary wl-refile
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elmo-signal wl-highlight wl-vars wl-version elmo-msgdb modb
modb-generic modb-entity luna mime elmo-util emu invisible inv-23 poem
poem-e20 poem-e20_3 eword-decode std11 elmo-date elmo-vars
elmo-version w3m-load mime-w3m w3m browse-url doc-view jka-compr
image-mode w3m-hist w3m-fb bookmark-w3m w3m-ems w3m-ccl ccl
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mm-view mml-smime smime savehist lawlist-vr-hr lawlist-whitespace
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esh-io esh-cmd esh-opt esh-ext esh-proc esh-arg eldoc esh-groups
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Memory information:
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