A ref to the textarea to measure.
Backed by one persistent hidden mirror div (created on mount, removed on unmount) that copies the textarea's font / padding / border / wrap so measurement matches what the user sees — including zoom, since the grid applies zoom via font-size (not a transform), which the computed style already carries.
Caret geometry for a
<textarea>: turn a caret offset into a viewport rect you can anchor a float (dropdown / function tip / range hint) to. The workhorse behind the formula editor's popups; also the measurement seam the token-highlight overlay reuses. DOM-only (no SDK), so it lives here alongside the other generic hooks and is shared by both the inline cell editor and the formula bar.