[CHTML/Fira] Extra spacing after \underbrace group and subsequent characters

Issue Summary

When using mathjax-fira font with CHTML output, there is unwanted extra spacing between an \underbrace group and the
character that immediately follows it. This gap does not exist in MathJax v3.2.2 (default font)

The issue is most visible when a digit follows a closing \underbrace, e.g., \underbrace{0000\cdots0}3 — the "3" appears visually detached from the underbrace group.

Steps to Reproduce:

1. Configure MathJax v4 with mathjax-fira font and CHTML output

2. Render the following LaTeX:
   $$A = 0.\underset{80\text{ 个 }0}{\underbrace{0000\cdots0}}3,\quad B = 0.\underset{120\text{ 个 }0}{\underbrace{00\cdots0}}2$$

3. Observe the gap between the underbrace group and the trailing digits "3" and "2"

Expected behavior:

The trailing digits should appear immediately adjacent to the underbrace group with no extra spacing, consistent with MathJax v3.2.2 rendering.

Comparison screenshot:

• Left (v3.2.2): No extra spacing — digits "3" and "2" are tight against the underbrace content
• Right (v4.1.2 + Fira): Visible gap before "3" and "2"

Technical details:

• MathJax Version: 4.1.2

• Client OS: macOS

• Browser: Chrome (also reproducible in Safari/Firefox)

• Font: mathjax-fira

• Browser: Chrome (also reproducible in Safari/Firefox)

• Font: mathjax-fira

I am using the following MathJax configuration:

MathJax = {
  output: {
 font: 'mathjax-fira',
  },
  tex: {
 inlineMath: [['$', '$'], ['\\(', '\\)']],
 displayMath: [['$$', '$$'], ['\\[', '\\]']],
  }
};

and loading MathJax via

<script src="https://cdn.jsdelivr.net/npm/mathjax@4/tex-chtml.js"></script>

Supporting information:
- The issue does NOT occur in MathJax v3.2.2 with its default font
- No console errors are produced

Start by reproducing the reported expression in MathJax v4 with CHTML output and the mathjax-fira font, then compare it with MathJax v3.2.2 using the same expression. Focus on the spacing between the closing underbrace group and trailing digits; done means the digits render immediately adjacent without the extra gap.