\phantom seems to be under-calculating width

Issue Summary

I've noticed that \phantom{} is producing content that seems to be too narrow.

Steps to Reproduce:

1. At the demo site, try:

$$(1+2+3)$$

$$(\phantom{1}+\phantom{2}+\phantom{3})$$

I see a few things. Mainly, the space where phantom elements are is too narrow for what is phantomed out. There is also some asymmetry in the example above, where there is more space for the \phantom{1} than for the \phantom{3}. If anything, it should be the other way.

It does not seem to change anything if I wrap the \phantom instances in \mathord{}. In other words, it seems the missing space is due to something more than the spacing that surrounds the binary operators.

Technical details:

• MathJax Version: 4.1.3 (at the demo site)
• Client OS: (e.g., Mac OS Tahoe 26.5.2)
• Browser: (Firefox 152.0.6)
• I see this with both CHTML and SVG math rendering.

Reproduce the reported expressions on the MathJax demo site in both CHTML and SVG output, comparing normal content with \phantom content. Trace the rendering entry point for \phantom and verify that the phantom widths match the hidden expressions and preserve the reported spacing symmetry.