Text runs are serialized without \text{} when defaultMode is "text", making the value unusable outside the mathfield #3078

Description

In a mathfield with defaultMode: "text", a text run is serialized without a \text{} wrapper, while a maths run is wrapped in $ … $:

typed:   Hello  →  Ctrl+M  →  x^2  →  Ctrl+M  →   world
mf.value:  "Hello $ x^2 $ world"

That value is only meaningful to another text-default field. Handed to anything else that reads LaTeX — convertLatexToMarkup(), renderMathInElement(), a server, a database — Hello is a run of five italic variables and its spaces are gone, because outside a text-default context unmarked words are maths.

It round-trips within a text-default mathfield, so it is self-consistent; the problem is that the emitted LaTeX is not portable, and there is no option to ask for the marked form.

Steps to reproduce

<math-field id="mf" default-mode="text"></math-field>

1. Type Hello
2. Ctrl+M, type x^2, Ctrl+M
3. Type world
4. Read mf.value

Actual: Hello $ x^2 $ world
Expected: \text{Hello }x^2\text{ world}

Then convertLatexToMarkup(mf.value) renders Hello as italic variables run together, rather than as words.

Reproducible without a browser, which makes it easy to test:

const atoms = parseLatex('Hello $x^2$ world', { parseMode: 'text' });
Atom.serialize(atoms, { defaultMode: 'text' });
// master:   "Hello $ x^2 $ world"
// expected: "\\text{Hello }x^2\\text{ world}"

Cause

The two mode serializers make opposite decisions from the same flag:

• src/core/modes-text.ts — needsWrap is options.defaultMode !== 'text', so a text run is never wrapped in a text-default field.
• src/core/modes-math.ts — a maths run is wrapped in $ … $ precisely when options.defaultMode === 'text'.

So in a text-default field the maths is marked and the words are not. That is fine while the whole document is text, and ambiguous as soon as it is mixed.

Proposed fix

A text run that shares the document with maths is marked whatever the field's default mode; words on their own are left alone. The decision is made where a run is already being split by mode, so both the styled and skipStyles paths agree.

PR: #3079

Existing suite passes unchanged: 338 tests, 252 snapshots. The three new cases fail without the change.

Environment

• MathLive 0.110.0, and master at c952a20
• Chrome 141, macOS

Why it matters

This is the blocker for the use case in #2420. An application that stores a mathfield's value and renders it anywhere else — a document, a print view, another mathfield in maths mode — has to re-mark the words itself to know which parts were prose. We do that with a normalization pass over every value on the way in and out; the marked form removes the need.