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I'm wondering how a modern browser handles scrolling of a displayed page.

Is the whole page re-rendered after every scroll step, or is there an internal bitmap or possibly even GPU accelerated buffer that is just moved around?

There is a lot of talk about GPU acceleration these days, but what exactly is accelerated: The paiting process of the actual content (images, text, lines...), the compositing of the various elements on the page, or both?

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closed as not a real question by Linker3000, Gareth, Mokubai, Nifle, Simon Sheehan Nov 8 '11 at 21:17

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Doesn't need to be a particularly modern browser. Windows (the OS) has supported scroll support for quite a while, for all applications. Windows keeps track which part of a window are "valid" and which are "invalid", and asks the program to redraw invalid parts. After scrolling, only a thin edge is invalid and needs redrawing.

(Current versions of Windows use a function called ScrollWindowEx for this)

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Does that mean that the API user only has to worry about redrawing the parts that have "scrolled into view", and the OS moves all the remaining parts up/down without any application layer repaints? – lxgr Nov 7 '11 at 12:23
Yup. And you needed that last code anyway. Normally Windows asks you only to draw the visible part of your window, so when that visible part changes (e.g. when a forground dialog window is closed) you can get a request to draw that newly visible part. – MSalters Nov 7 '11 at 16:12
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