ConstA WIDGET, not a dialog page: it takes one function, renders no buttons, and treats workbook /
home as optional, so the same panel serves the Insert Function wizard's second page and (later) a
debug pane in the script editor, where a custom function being written can be filled in and watched
evaluate.
TEXT is the state of record. Assembly is string concatenation, and an argument mid-typing
(A1:) has no IFormula at all — which is most of the time this panel is being used. Each row's
text is seeded into its field ONCE and never echoed back: FormulaField's value is a committed
baseline that re-derives the display text whenever it changes, so feeding a keystroke back through
it would re-serialize under the caret (A1 → =A1).
Each argument evaluates on its own, through the IFormula its own field parsed — never as a
subtree of the assembled call. One blank or half-typed argument fails that parse, and every row's
value would blank at once, during the exact activity this panel exists for. Only the result line
evaluates the whole call.
Excel's Function Arguments — the function's name and summary, then one labelled field per argument (each showing what it evaluates to), the focused argument's description, and the assembled call's result.