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.
