Verified: usage-sessions-agents-fit 8/8 runs green in Chromium and WebKit, en and zh; also at 440px (a copy with 860/440); sess-agent-strip, sessions-toolbar and sessions-usage-open pass. sessions-overview's WebKit en case fails 2 runs in 3 on its own, with "ResizeObserver loop completed with undelivered notifications" caught as a page error, a different cause, not in this commit.