Public API to get computed metrics (width/height/depth) without rendering to the DOM #3077

Is your feature request related to a problem?

Any app that needs to know how big a LaTeX expression will render — before or without inserting it into the visible page — currently has to call convertLatexToMarkup(), insert the resulting markup into a hidden/off-screen element, force a layout, and read the size back with getBoundingClientRect(). This works, but it costs a synchronous forced reflow per measurement, and it means keeping a hidden DOM mirror whose styling (font, size, weight) has to stay in exact sync with wherever the real content renders.

What I found looking at the source

Internally, MathLive already computes exact width/height/depth for every box using font-metrics tables (src/core/box.ts, getCharacterMetrics, horizontalLayout) — entirely without touching the DOM. That's genuinely the hard part (accurate math typesetting metrics), and it's already solved here. The computed numbers just aren't exposed by any public export — convertLatexToMarkup and friends only return markup, not the dimensions that went into producing it.

Describe the solution you'd like

A small public function, something like:

function getLatexMetrics(latex: string, options?: SomeOptions): { width: number; height: number; depth: number }

that returns the box tree's already-computed dimensions directly, with no DOM node created. Naming/shape totally open to whatever fits MathLive's existing conventions better.

Who would use this

Anyone doing custom layout around math content — canvas/SVG-based renderers, print layout, custom editors — that needs a size before deciding where/how to place the expression, without paying for a DOM round-trip for a number MathLive already calculated internally.

Additional context

Happy to put together a PR if this direction seems reasonable — wanted to check on the API shape first per the contributing guide.