The custom sidebar read as an ad-hoc widget: collapsed rows were the
expanded layout clipped to the strip, the libraries tree used a
staircase of indents and mixed row heights, and hover expansion pushed
the whole content area sideways.
Rebuild the rail on M3 Expressive geometry: collapsed destinations are
icon-only 56x32 pills centered in an 80px strip (48px icon-only on TV),
expanded rows are fixed-height destinations with a full-width stadium
indicator, and the two layouts crossfade/lerp during the width morph
instead of clipping. The libraries section is flattened into full-width
destination rows under a chevron header at standard destination
metrics, hover/touch expansion overlays content as a rounded, shadowed
panel over a dimmed scrim instead of pushing it, and rail width, item
morphs, content translate, and bleed all share one duration/curve.
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