NotificationBell mobile sheet renders off-screen under a backdrop-filter ancestor (unportaled trigger variant)

[ditto-m]

Summary

<NotificationBell>'s mobile bottom sheet (the default variant="trigger", used per the README recipe <NavigationBar trailing={<NotificationBell items={items} />} />) renders squeezed near the top of the screen instead of docked to the viewport bottom, when NavigationBar has picked up the Liquid Glass .etu-glass material (the default — both .etu-navbar and .etu-mtb opt into it per the README).

Root cause

.etu-notif-bell-sheet is position: fixed; bottom: 0. Per the CSS spec, backdrop-filter (like transform/filter/perspective/will-change: transform) establishes a new containing block for position: fixed descendants. .etu-glass sets backdrop-filter: blur(24px) saturate(180%), so once the sheet's ancestor chain includes a .etu-glass element (here, .etu-navbar.etu-glass, sticky at the top), the sheet's bottom: 0 resolves against that ~48px-tall navbar box instead of the real viewport — the sheet ends up mostly clipped above the visible screen.

The row variant already avoids this: NotificationBell's isRow panel is portaled to document.body via createPortal (see the "Popovers inside the sidebar must be portaled" note in the README). The default variant="trigger" panel — including its mobile bottom-sheet form — is not portaled, so it inherits whatever containing block its mount point happens to sit inside.

Repro

1. Mount <NavigationBar trailing={<NotificationBell items={[]} push={{...}} />} /> per the README's documented mobile recipe.
2. Open at a mobile viewport (<720px) with a non-empty push (so the row has content) or any items.
3. Click the bell trigger.
4. The sheet renders with most of its height above y=0 instead of sliding up from the bottom.

Confirmed via getBoundingClientRect() + walking ancestors for transform/filter/backdrop-filter/will-change/contain — .etu-navbar.etu-glass's backdrop-filter is the containing-block trigger.

Suggested fix

Portal the trigger-variant's mobile sheet (and arguably the desktop popover too, for the same reason) to document.body, the same way the row variant already does — rather than relying on the consumer's ancestor chain never setting backdrop-filter/transform/filter.

Workaround in place

etamong-playground/res-train (planning#1149 / #1147 adoption PR) drops .etu-navbar's backdrop-filter specifically at the mobile tier (@media (max-width: 719px) { .etu-navbar.etu-glass { backdrop-filter: none } }) until this portals upstream. Losing the glass blur on the mobile top bar is an acceptable trade until fixed here.