Tainted form example appears to break the browser history stack

[fnimick]

I have confirmed this behavior in desktop Firefox and Chrome. I'm not sure whether this is a problem with this specific example, or tainted form handling more broadly, but I have looked at the example and can see no special case behavior causing this.

To reproduce:

Navigate to the tainted form field example page, then to another page, then back.

Observe the state of the history stack. In my case:
Back stack:

Forward stack:

Then, attempt to navigate back from the tainted form, and click "confirm" on the modal.

Observe that rather than navigating back, instead the most recent history page has been navigated forward to, adding a new entry to the back stack. The forward stack has been cleared and is now empty.