How can I properly fix the vertical alignment of the tombstone (QED sign) in list-proofs?

Question

I frequently encounter the following situation: I am proving a claim which consists of multiple sub-claims, which I've enumerated in a list using the enumerate environment provided by the eponymous package. The proof then also takes the form of such a list. At the end of the proof, the tombstone (a.k.a. QED symbol) appears vertically offset from the final line of the last item of this list. Here's a minimal working example to see this behavior:

\documentclass{article}
\usepackage{enumerate}
\usepackage{amsthm}
\begin{document}
\begin{proof}
\begin{enumerate}
\item Problem
\end{enumerate}
\end{proof}
\end{document

I don't like this behavior, and would like the tombstone to appear vertically aligned with the final line of the last item. So far, I've gotten by with some quick and dirty solutions (using negative \vspace) but I've now come across a case where that doesn't work (because of page breaking). So I'm looking for a proper solution.

I've taken a look at this question, which the site's algorithm suggested to me while typing this up, but didn't like the quick and dirty way suggested there (and wasn't able to implement egreg's answer in a satisfactory manner).

Answer 1

No problem if you load ntheorem with options [amsthm,thmmarks]:

\documentclass{article}
\usepackage{enumerate}
\usepackage[thmmarks, amsthm]{ntheorem}
\begin{document}
\begin{proof}
\begin{enumerate}
\item Problem A.
\item Problem B.
\item Problem C.
\end{enumerate}
\end{proof}
\end{document}