PR #22941. The thread wrapper capped at a hand-counted calc(100vh - 220px); the chrome it counted was retired, leaving a 62px band at desktop. The cap is now measured. Stage TEST data (The Willows), local dev server, headless Chromium.
before: 94px under the thread (62 dead + 32 padding) / after: 32px padding only
before: 94px / after: 32px
before: 64px (40 dead + 24 padding) / after: 24px
