ConstThis is the "B" half of the formula-editor hint UI (the autocomplete list is "A"). Presentational
and props-driven — the editor owns the caret → hint derivation and passes it down. The expand/collapse
state persists (via usePersistentState); close is delegated via
FunctionPreviewPanelProps.onClose. No Example is shown: IDescriptor has no function-level
example and we won't fabricate one. The active-argument emphasis is driven by hint.argIndex. Nothing
here mutates the edit.
The function preview panel shown while editing a formula, modeled after RowZero/Excel Online: a collapsible FunctionPreviewHeader (the signature with the active argument highlighted, plus an accordion collapse and an optional close) over the FunctionPreviewContent body (description + stacked argument list). This component is just the composition — the two halves are exported so the body can be reused as a hover tooltip independent of the collapse chrome.