Layout switch — accessibility

Keyboard

[ toggles the menu (full ↔ rail) everywhere in the app, whether or not any other segment is registered.
] toggles the context panel, but only when a screen registered one; otherwise the key is a no-op — there is nothing to toggle.
Both keys are ignored with a modifier held (metaKey/ctrlKey/altKey), when the keydown event is already defaultPrevented, while typing in a field (input, textarea, select, or any contenteditable — including a rich editor's body), and while any overlay (dialog, popover, drawer) sits above the shell.
Every button in the pill is a real <button>; Tab reaches them in DOM order (menu → list → panel).

The pill is role="group" with one aria-label for the whole cluster ("Layout") — a screen reader announces it once, not as three unrelated buttons.
Every button sets its own aria-label; none relies on the native title fallback for its accessible name. The DS Tooltip (not title) provides the visible hint, wired through aria-describedby on the button it wraps.
The list-layout and context-panel buttons carry aria-pressed — genuine two-state toggles. The menu button carries aria-expanded instead, and no aria-pressed: it is an action that switches the whole sidebar, not a state of itself.
The divider between segments (.pp-ldiv) is aria-hidden="true" — purely visual, never focusable or announced.

Foundation ring from ds.css applies to every button; the pill declares no focus style of its own.
Toggling a segment does not move focus; the button that was activated keeps it, so most screen readers re-announce its new aria-pressed/aria-expanded state on their own.
The pill is not a focus trap and never steals focus on mount.

Computed from packages/ui/src/tokens/tokens.css:

Rest icon colour --fg-secondary on the pill's --surface-100: 3.93:1 light, 6.20:1 dark.
Hover/active/pressed icon colour --fg on --active-surface: 15.29:1 light, 11.41:1 dark.
Pill border --border on --surface-100: 1.15:1 light, 1.22:1 dark — decorative only, the shape does not depend on the hairline being visible against every background.
Focus ring --accent: 4.44:1 light, 3.83:1 dark (same token as the rest of the shell).

Nothing is announced when a segment appears or disappears (a screen registering or unregistering support) — a screen-reader user discovers the change only by tabbing to where the pill sits.
The tooltip's keyboard hint is baked into the same string as the accessible name ("Collapse menu to icons ( [ )"); it is verbose for a screen reader on every focus, but keeps the hint and the name from drifting apart (see usage doc, Content).