Today the OPTIONS author view has two static headings — DEFINITION and VALUE — and a small icon DEFINE / FILL toggle floated to the top-right. The proposal collapses all three into a single real segmented control that sits on the column divider: its left segment titles the definition column, its right segment titles the value column, and clicking a side is the DEFINE ⇄ FILL switch (the greyed column + DEFAULTS wash follows, exactly as now). An info (i) flanks each side — one explains DEFINITION, one explains VALUE — each tinted like the segment it sits by: the active side takes the toggle accent, the inactive side goes muted, swapping as you toggle. Drag the handle to check narrow widths; the toggle stays pinned to the divider.