Restoring missing space before large parentheses in `newtxmath`?

Question

As shown by the difference between the two lines of the image below, for sufficiently large pairs of parentheses in newtxmath, the space in front of the left parenthesis disappears. This results in spacing that is more cramped than desired. Is there a clean (i.e., without manually adjusting the kerning each time) way to restore this space?

Here is the code used to generate that image:

\documentclass{minimal}

\usepackage{newtxmath}
\usepackage{mathtools}

\DeclarePairedDelimiter{\paren}{\lparen}{\rparen}
\newcommand*{\tall}{\int_0^{t^2} x\,dx}
\newcommand*{\taller}{\sum_{\substack{(k,m)=1\\0 < k < m}} k}

\begin{document}

\begin{align*}
  &\exp\paren*{\tall} \\
  &\exp\paren*{\taller}
\end{align*}

\end{document}

Answer 1

I prepared two standalone documents, doing \fbox{\Bigg(} first with Computer Modern, then with newtxmath, setting \fboxsep to 0pt and \fboxrule to 0.1pt in order to precisely show the bounding box.

The command \Bigg works exactly the same in text as in math. Next, I did \includegraphics with both outputs, getting

(left CM, right NewTX).

Next experiment, I typeset $\exp\paren[\Bigg]{}$ in both standalone documents.

You can note that the difference in the bounding boxes has an important role.

The difference in the bounding boxes is even more outstanding for really bigger parentheses. Here's the experiment: I define

\makeatletter
\newcommand{\Bigggg}{\bBigg@{4.5}}
\makeatother

and retry the first experiment, getting

Again, left is Computer Modern, right is NewTX. The size is slightly smaller in the latter case, but this shows the problem is in how the bounding box of the composed parentheses are defined.

Answer 2

When you use \left and \right, TeX inserts additional spacing around the delimiters. A command declared with \DeclarePairedDelimiter does not. Therefore, if you wanted the extra space after an operator, you could switch to \left( and \right).

You could also define a command that expands to \,\paren{#1} and use that.

I know I say this a lot, but this is another one of the many, many problems that goes away if you switch to the modern toolchain. unicode-math with TeX Gyre Termes Math looks very similar to newtxmath, but does not have this bug.