Drop the fullscreen panel's whole-box no-drag: it swallowed the strips'
blank-run window drag and made no-drag the steady state, turning the
open/close app-region nudge into a computed-value no-op (electron#32341)
exactly when a hidden panel opens straight into fullscreen. The panel now
declares no steady-state app-region in either presentation, and ui-dockkit
subtracts the one box base.css never matches — the plain-div split divider —
alongside tabs, strip-end chrome, and pane bodies. New specs pin both
contracts against the stylesheets on disk.