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:

 1. Disable all javascript on your browser.
 2. (Re)load an appropriate Stack Exchange page (or any long page with a `<textarea>`).
 3. Focus the textarea for keyboard input but use the mouse to scroll it out of view.
 4. Start typing.
 5. The *browser* will then scroll the textarea into view. (Not verified on IE.)
 6. The page can't be doing it, since all javascript is shut off.


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**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.