bug#14721: slow scrolling on windows 7

[Mario Valencia <mariovalspi@gmail.com>]

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Well I guess that's it then, there's no way to fix it. :(


2013/6/28 Eli Zaretskii <eliz@gnu.org>

> [Please keep the bug address on the CC list.]
>
> > Date: Thu, 27 Jun 2013 17:55:15 -0500
> > From: Mario Valencia <mariovalspi@gmail.com>
> >
> > I don't know why scrolling has to be so slow. I certainly don't have that
> > problem with any other text editor.
>
> Emacs have a lot of display-related features other editors don't,
> which makes the simplest display-oriented operations very complex
> under the hood.
>
> Just to give you an idea of the complexity: when text includes
> variable-size fonts and images, how do you know where is the start of
> the next or previous window-full, in terms of character positions?
> E.g., if the next/previous line beyond the current window is a large
> image, scrolling would mean to move the viewport by a single line.  By
> contrast, if you are in some kind of folding mode, which hides a lot
> of text, the next/previous window start can be very far away.  There's
> no way Emacs can know this in advance, so it actually scans the text
> before and after the window to decide where to start the next
> window-full.
>
> > Moreover, it scrolls just fine with the scroll bar.
>
> Scroll bar scrolls by smaller amounts, so Emacs succeeds to optimize
> redisplay and keep up.
>
> > And why is scrolling up with PgUp much slower than scrolling down?
>
> For boring technical reasons, scrolling up and down use very different
> code paths in the display engine.
>

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