There is 2-3 related bug symptoms which I'm pretty sure all have the same root cause:

1) If you load a page then the standard browser keyboard up and down arrows do not scroll the page up and down. Likewise the page up and page down buttons do not work either. If you happen to click into the page body then they now do work but then if you click outside the page but still inside the browser window then you can end up back in the default broken state.

2) Scroll state restoring on reload is broken. ie if you load a page, then scroll down a page, then reload it, then in almost all browsers on normal pages your scroll state is restored to where it was. This doesn't happen and you end up back at the top.

3) Depending on different themes or plugins that add bits to the top of the page you can end up with double scrollbar, one on the page div and then the second on the actual document.

I haven't quite reverse engineered when or where the bug has come in but it looks like 4.0 only and I'm assuming it is related to the scroll region only being a div in the page rather than the whole actual page like it was previously.

It is a real bug in prod but it is especially worse in development because you often reload the page over and over and you change things and you always need to re-scroll down to where you were before.