The open tab and its panel share one glass surface and one outline, like a folder tab: the panel card draws no border of its own, the rail's ::before draws the outline with a notch at the tab, the tab closes the shape with its own borders and two concave fillets round the inner corners. The strip publishes the open tab's extent and the panel's height on the rail; a short panel grows to reach its tab. Cyber joins square below its chamfer. Unselected tabs are plain labels.
Co-Authored-By: Claude Fable 5.1 <[email protected]>