BackButton: visible vs aria-only label branches, custom class
application, history.back() click handler.
OverflowPillButton: +N pill render, aria-expanded matrix
(closed default → open after click), per-person link rendering with
correct href, Escape closes the dropdown.
Both are reused widely; their coverage closes the line and function gap
left after the DocumentTopBar split inflated the denominator.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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