Root cause: class switching (px-2↔px-3, justify-center↔gap-2.5,
conditional DOM rendering) caused instant layout jumps during the
200ms width transition.
Fix: internal layout is now 100% static — same padding, same gap,
same DOM structure in both states. Only opacity transitions on text
labels (via Label component). The sidebar overflow:hidden + width
transition handles the visual collapse. Collapse icon rotates 180deg
instead of swapping between two icons.
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