A pane of glass behind child, after iOS's Liquid Glass, a pill unless
clipper shapes it. It casts shadows only outside itself, as one
under it would show through. Without Impeller's shader filters it frosts
but does not bend.
Draws the panes below from one capture of what lies behind them, taken as
the frame is composited, in place of each pane reading the backdrop
(.agents/architecture.md). Only one may enclose a pane.
Dims what it covers by color, under child, as a modal barrier does. An
even scrim blurs to itself, so a pane above takes it from the shader rather
than its capture, and a scrim fading renews none.