Review findings from the first round:
- ui-settings renders its full-viewport panel as a fixed-position DESCENDANT
of the sidebar column, so `pointerleave` never fires when the pointer moves
onto it and the bars stayed drawn after it closed. Leaving is now decided
against the column's box from a document-level pointermove that exists only
while the bars are drawn; the element's own leave stays for the pointer that
leaves the window.
- The rebind gate judges the rule rather than each declaration, so a pair that
mixes `transparent` with an l2 hover no longer passes, and the elevation
half compares whole values instead of token shape.
- Hiding no longer exempts a sheet from the elevated-surface rebind check.
- The e2e polls the reveal before reading a colour for the golden, and pins
that a pointerless scroll draws no thumb.
The session list overflows after a handful of sessions, and its scrollbar
was drawn permanently in a column that is at rest most of the time.
SidebarRoot now tracks the pointer over the whole column and rebinds
ui-theme's scrollbar indirection pair to `transparent` while it is
outside, keeping the thumb for 2s after the pointer leaves so it does not
blink out on the way past. Rebinding colour leaves the list's
`scrollbar-gutter: stable` reservation in force, so revealing the bar
moves no row.
ui-theme's gate now states the widened contract: a rebind targets an -l2
token pair or `transparent`, and nothing else.
creatixchu