fix: Enable user selection for math static elements

[tsuji-riya]

I would like to enable user selection on <math-span /> and <math-div /> elements.

To do this, I removed user-select: none from the .ML__latex CSS class in core.less, so users can now select and copy the math content.

But this had a side effect: the _mathMLContainer (a hidden MathML element used for screen readers/accessibility) also became selectable, causing the same math expression to be duplicated when copied. To fix this, I added user-select: none directly on _mathMLContainer in math-static-elements.ts, so only that element is excluded from selection/copy while the visible math stays selectable.

[arnog]

Thank you for this PR, and for the clear description. Making the content of
<math-span> and <math-div> selectable is a reasonable request, and you were
right to catch the duplicate caused by the hidden MathML container.

I tested the change in Chrome with \frac{a}{b}+x^2+\sqrt{y}, and I found some
problems that prevent me from merging it as is:

1. The .ML__latex rule is shared. It applies to the mathfield, to
   renderMathInElement(), to the server-side rendering output and to
   mathlive-static.css, not only to the static elements. Removing
   user-select: none changes all of them.

2. renderMathInElement() now copies the expression twice. The fix for the
   duplicate covers _mathMLContainer, but the .ML__sr-only span created in
   static-render.ts has the same problem.

3. A page selection that crosses a <math-field> now highlights and copies
   the content of the field. On iOS, removing -webkit-user-select: none may
   also enable the long-press selection callout inside the field. I have not
   tested this on a device.

4. The copied text is not usable. The selection follows the layout boxes,
   so the expression above is copied as b a +x 2 +√ y, with a line break
   between each part. The denominator comes before the numerator and the
   structure is lost. Before the change, the copy gives ab+x2+y from the
   hidden MathML, which is at least in reading order.

A narrower approach would give you what you need without these side effects:

• Keep user-select: none on .ML__latex in core.less.
• Enable selection only inside the shadow root of the static elements, for
  example with a rule on [part="render"] .ML__latex.
• Add a copy event handler on the static elements that puts the LaTeX source
  on the clipboard, so that pasting gives \frac{a}{b}+x^2+\sqrt{y}.

Would you be willing to update the PR in this direction? A Playwright test for
the copy result and a CHANGELOG entry would also be welcome.

Thanks again for the contribution.

[tsuji-riya]

Thanks for the detailed review! 😃
I've updated the PR as you suggested:

• user-select: none is restored on .ML__latex. Selection is enabled only in the shadow root of the static elements ([part="render"] .ML__latex), so the mathfield, renderMathInElement() and SSR output are unchanged.
• A copy handler puts the LaTeX source on the clipboard. When surrounding text is included, the LaTeX is wrapped in $...$ / $$...$$. If the selection includes a <math-field>, the default copy behavior is used.
• Added Playwright tests and a CHANGELOG entry.

Let me know if you'd like any changes.