mouse-drag.el and automatic-hscrolling interact poorly

mouse-drag.el and automatic-hscrolling interact poorly

[Paul Du Bois]

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In GNU Emacs 21.2.1 (i386-msvc-nt5.1.2600)
 of 2002-03-19 on buffy
configured using `configure --with-msvc (12.00)'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENU
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: nil

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

;; eg

(require 'mouse-drag)
(define-key global-map [M-down-mouse-1] 'mouse-drag-drag)
(setq automatic-hscrolling t)
;; switch to an interesting buffer
(setq truncate-lines t)

;; 1. put point in the middle of a line
;; 2. do a leftward M-drag
;; 3. Notice that as the cursor approaches the left edge of the
;; screen, the hscroll resets to 0

A quick fix is to modify the (let (...) ) in mouse-drag-safe-scroll to
temporarily unset automatic-hscrolling.  I don't know whether that's a real
fix or just a workaround -- I don't understand this comment in scroll-right:

   If `automatic-hscrolling' is non-nil, the argument ARG modifies
   a lower bound for automatic scrolling, i.e. automatic scrolling
   will not scroll a window to a column less than the value returned
   by this function.

p