The hidden mirror.
To anchor the popover at the caret, the lib runs a hidden <div> next to the textarea — same font, padding, border, line-height — and reads its layout to find the caret's pixel coordinates. Below, the mirror is exposed so you can see what it sees.
Browsers don't expose textarea caret coordinates directly.
There's no textarea.getCaretRect() API. Range objects only work on contenteditable. The mirror is the general way to compute caret pixels for a textarea — you build a parallel layout that the browser can measure, then read offsets off a zero-width span at the caret position.
The technique predates React: it ships in textarea-caret-position (MIT, 2015) and has been battle-tested across nine years of browser quirks — the Firefox overflowY workaround, the end-of-line non-empty-character contract, the property list a mirror must replicate. The lib ports the math to TypeScript and narrows it to <textarea> only; contenteditable support lands in v0.2 via the Range API instead.