This doesn't have anything to do with Stack Exchange pages, it's a browser thing and mostly a very good thing.
You can see that the page isn't doing it by:
- Disable all javascript on your browser.
- (Re)load an appropriate Stack Exchange page (or any long page with a
<textarea>). - Focus the textarea for keyboard input but use the mouse to scroll it out of view.
- Start typing.
- The browser will then scroll the textarea into view. (Not verified on IE.)
- The page can't be doing it, since all javascript is shut off.
Possible workaround:
You can write a userscript or browser extension that applies position: fixed; to the WMD textarea and shrink it to a smaller fixed box on the screen.
That way, when you type, it will not change the scroll-position of the rest of the page/screen.
With a little extra work, the script could also check for inserted/changed "preview" elements, and attempt to keep them sensibly scrolled in view.