What happens:

1. Edit a long page
2. Scroll to the middle of the page and modify some text
3. Click Preview
4. Observe that in your edit window, you have been returned to the top of the page's wikitext. To get to the section that you were editing, you need to scroll or do a Find on the page.

What should happen:

1. Edit a long page
2. Scroll to the middle of the page and modify some text
3. Click Preview
4. In the edit window, you should see the wikitext that you were modifying, not the top of the wikitext.

The 2003 wikitext editor handles this situation just fine.

I was editing with the 2010 wikitext editor's syntax highlighting turned on, which may be relevant.