When turning Dark off (Dark -> Light transition), the NEW layer is Light.
Previously we tried shrinking the OLD layer (Dark) over it, but if the
root snapshot captured a transparent html, revealing the transparent layer
produced no visual change.
Now we:
1. Always keep the NEW layer on top (z-index: 2).
2. Always animate the NEW layer expanding from 0 to max radius.
3. Explicitly set background-color on html in input.css to ensure the
VT root snapshots are fully opaque, avoiding transparent captures that
make expansions invisible.
4. Add background: var(--color-paper-theme) directly to ::view-transition-new
just to be absolutely certain it's painted opaque.
xfy
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