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Re: strange scrollbar behaviour

From: Jan D.
Subject: Re: strange scrollbar behaviour
Date: Sat, 04 Oct 2003 20:53:38 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030921

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

I'm running the latest CVS code on Windows XP.  I don't know whether
this happens on X Windowing System builds.

Here's how to reproduce the problem:

* size a window to be 20 lines tall

* switch to a new buffer in that window

* "C-u 30 RET C-u 30 ." (insert 30 blank lines followed by 30 dots)

* use the scrollbar to scroll up and down slowly and notice the line
  of dots jumping up and down seemingly at random, as opposed to
  following the scrollbar's movement.

I think this is an artifact caused by the fact that Emacs uses character based scrolling instead of pixel or line based scrolling. These kind of effects can be seen when there are mix of a lot of short lines followed by long lines. Basically Emacs thinks the first 30 lines is 50% of the buffer displayed, and the 30 dots is the other 50%. But when counting lines, this very far from the actual ratio.

On X I see different behaviour depending on what toolkit is used. This is because scrolling is not the same for all toolkits.

        Jan D.

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