#1381 landed the bar's horizontal position; this branch decides when it is
drawn. The e2e keeps both scenarios and the golden carries both palettes'
pointer-state readings alongside the new edge-offset lines.
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.
Review found three scroll containers sitting on surfaces the rebinding
contract covers, none of which rebound: ui-primitives' shared Menu card
on --dsw-specific-menu (the surface PopupSelectView already rebinds for),
and the composer input and question composer cards, both on
--dsw-specific-input-major. Each rendered the l1 thumb, which differs
from l2 only in the dark palette and only on that surface, so a
light-palette screenshot and a code read both look correct.
Adds the mechanical check that would have caught them instead of leaving
it to inspection: a sheet that scrolls somewhere and paints a known
elevated surface somewhere must rebind. The elevated set is derived from
the sheets that already rebind, since a rebinding rule paints the surface
whose elevation it declares, so a new elevated surface joins the set by
rebinding rather than by anyone updating a list. Surface-level rather than
element-level because the card and the descendant that scrolls are
separate rules and CSS text does not say which contains which. Verified by
reverting each of the three fixes in turn: the check names the sheet and
the surface every time.
Also commits snapshots/sidebar-scrollbar/geometry.expected.md, the
resolved scrollbar style and geometry in both palettes. The aria goldens
the other web scenarios commit cannot carry a CSS-only change, since it
alters no DOM and no accessible name and leaves their trees
byte-identical. Absolute coordinates stay out: they track font metrics and
the laid-out sidebar width, so committing them would document the platform
and force a per-platform re-record.
