Fixes the overlap half of #4339: an equation with \tag{…} that is wider than the editor column is drawn underneath its own tag.

Reproduced

The reporter's formula in a math block, in a column narrower than the formula (a 520px viewport reproduces their screenshot exactly): on develop the tag (A.13) is painted over n^4(V_m − V_K) — 186px of overlap, measured as the last .base's right edge against the tag's left. With this change the overlap is 0 and the tag is reached by scrolling.

The scrollbar half of the report — a horizontal scrollbar under short inline formulas at any Max width percentage — was #5034 and is already fixed by #5466; inline-math-editor-width-5034.spec.ts covers it. (The comment's $\int k dx=kx+C $ no longer parses as math at all: pandoc's tex_math_dollars forbids a space before the closing $, adopted in #5449.)

Mechanism

KaTeX renders a tagged display equation as

<span class="katex-display"><span class="katex"><span class="katex-html">
  <span class="base">…</span>… <span class="tag">(A.13)</span>
</span></span></span>

with .katex-html { display: block; position: relative } and .tag { position: absolute; right: 0 }. .katex-html is a block, so it is as wide as the box around it, never as wide as the formula: the tag is pinned to the column's right edge while the white-space: nowrap formula overflows straight past it. Open upstream as KaTeX/KaTeX#1983.

The fix sizes that box to the formula and puts the tag in the flow:

.katex-display > .katex > .katex-html:has(> .tag) { width: max-content; min-width: 100%; }
.katex-display > .katex > .katex-html > .tag      { position: static; float: right; margin-left: 1em; }

• overflowing: width: max-content — the float lands after the formula, inside the scrollable extent.
• fitting: min-width: 100% — the float goes to the column's right margin, the numbered-equation look.

Both properties are needed. Floating the tag alone leaves the box at column width, so the float has nowhere to go and drops to a second line (measured: block height 72 → 112px, overlap 186 → 205px). width: max-content alone keeps the tag absolute, which just moves the overlap from mid-formula to its last 51px.

Scoped with :has(> .tag) so untagged equations — nearly all of them — keep their current box. :has() is already used in this stylesheet. muya never passes leqno/fleqn to KaTeX, so the left-tag variant cannot occur.

Two things this also fixes

• Same-line display math ($$…$$ inside a paragraph, feat(muya): support same-line display math #4935). Its popup shrinks to fit the formula, so its right edge is the formula's — the tag sat over the tail of even a short formula (23px overlap for a+b=c \tag{1}).
• The preview floated under an active math block, same structure.

The deliberate cost

A formula that fits is now centred in the space left of its tag, i.e. half the tag's width (~25px for (A.13), ~21px for (1)) left of the box centre. Exact centring and a tag that flows after the formula cannot both hold — this is the trade-off named in the upstream thread — and the overlap is the actual bug. Untagged equations are untouched, which is what the third spec asserts.

Test plan

packages/muya/e2e/tests/blocks/math-tag-overflow-4339.spec.ts, six cases: a block wider than the column, a block that fits, the active block's floating preview, an untagged block, and same-line display math both wider and narrower than the column. Four of the six fail on develop and pass with the rules.

• pnpm -C packages/muya/e2e exec playwright test --project=chromium tests/blocks/math-tag-overflow-4339.spec.ts — 6 passed; 4 fail with the CSS reverted
• Every other math spec: inline-math-align, inline-math-editor-width-5034, inline-math-overflow-4339, math-error-nowrap-4100, inline/same-line-display-math, inline/tex-math-dollars — unchanged
• pnpm -C packages/muya exec stylelint "src/**/*.css" — same 7 findings as on develop, none in the new rules
• Real app (pnpm run build:unpack, Electron): the rules ship in the bundled renderer CSS; a tagged formula wider than the 700px column scrolls with no overlap, one that fits keeps its tag flush at the right margin
• typing/math-and-diagrams.spec.ts — one pre-existing local failure ("clicking the rendered inline KaTeX activates the source for editing"), identical with and without this change

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