The composer seat is one node laid out against two different edges: in Chat it
is a sticky child of the column's scroller and rides its content box, which a
space-consuming scrollbar shortens, while a view declaring a composer overlay
gets an absolutely positioned seat against the padding box, which no bar
reduces. With the transcript scrolling the two tabs disagreed by the bar's
width, so the centred input card moved 4px sideways on every switch — and the
same 4px inside Chat when a growing transcript began to scroll.
The column now reserves its scrollbar gutter unconditionally and states the
overlay branch a scroll container on the same axes, so both states measure
against the same width.
#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.
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.
