\tag is expected to be always flushed at right when equation is overflowed

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- [x] Check the bug is reproducible in [the demo](https://katex.org). If not, check KaTeX is up-to-date and installed correctly.
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**Describe the pool rendering:**
For a long equation set to `overflow-x: auto`, the right-end `\tag{}` will overlap and scroll with the equation.
You can use Eqs. 7.85 and 7.86 as a test case (with a narrow window), available [here](https://spaceresearch.top/blog/97ae8555/#7-2-relevance-vector-machines).
```css
// allow katex equations scrolling horizontally, without vertical scrollbar
.katex-display > .katex {
height: auto;
overflow: hidden;
overflow-x: auto;
}
```

**Expected behavior:**
The `\tag` will be flush right no matter the equation is scrollable or not.
I'm not familiar with CSS. I tried to play around
```css
.katex-display>.katex>.katex-html>.tag {
position: absolute;
right: 0;
}
```
A feature of `position` mixturing `absolute` and `static` should be perfect. `sticky` doesn't fit in current setting, it is a mixturing of `relative` and `fixed` as I understand.

Reproduce the long-equation case in the KaTeX demo using Eqs. 7.85 and 7.86, a narrow window, and the provided overflow-x CSS. Inspect the rendered .katex-display > .katex > .katex-html > .tag behavior; done means the tag stays flush right whether the equation overflows or not.

Written by the indexing model from the issue text.