In TV mode the sidebar menu floated unreadably on top of the home rows,
the artwork and the settings list — two overlapping sets of text with no
panel behind either.
The rail's surface is transparent on TV so the full-bleed backdrop
continues behind a docked rail, which is safe because the shell displaces
content around it. 2.16.0 turned hover/touch expansion into an M3E modal
panel that overlays content at the collapsed offset instead of pushing
it, so there the rail painted nothing and content showed straight
through the menu.
Keep the transparency rule for every docked shape and paint the surface
whenever the rail floats, reusing the predicate that already gates the
overlay corner radius and edge shadow. The fade now shares the width
morph's duration and curve, so a shorter fade cannot strand a fully
grown, fully transparent panel over the content mid-collapse.
Reproducing needs Force TV mode plus a pointer, which is why this
surfaced on a SteamOS box driving a TV and never on Android TV's D-pad
path.
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