Variable MeasureModeConst

Custom-measurement callback for leaf nodes whose intrinsic size depends on their content (e.g. a text run whose width depends on wrapping).

The layout algorithm calls this with the available space along each axis and a mode describing how to interpret that constraint:

• 'undefined' — no constraint, return your natural size.
• 'exactly' — the constraint is exact, return that size.
• 'at-most' — fit within the constraint but you may report less.

The returned size is in terminal cells.