Add a blanket prefers-reduced-transparency gate to styles.css that sets
backdrop-filter: none everywhere, mirroring the existing blanket
reduced-motion rule. Frosted surfaces (composerDockCard via
composerSurfaceGlass, menus, popovers) re-sample their backdrop every
animation frame; on transparent plugin wallpaper themes a late filter
pass paints unblurred and the whole docked area strobes (issue 121910).
The alpha-mixed fills (--composer-fill at 88%) carry legibility on
their own, so nothing changes visually unless the OS accessibility
signal is on.
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