The 500ms setInterval in HudShell's band-measurement effect polled
forever, contradicting its own comment ("poll briefly until it exists,
then let the ResizeObserver own it") — the viewport was never actually
checked, so the timer never cleared. It kept re-running measure()
(DOM queries + getBoundingClientRect + a style write) every 500ms for
the life of the HUD window, one of several sustained per-window timers
reported in #98394 as sustained idle renderer CPU / repeated re-renders.
Extracted the effect into useHudTranscriptBand() (matching the
existing per-concern hook split in this file: useHudGlass,
useHudClickThrough, useHudThreadFocus) and made the interval check for
the viewport before re-measuring, clearing itself once found so the
ResizeObserver takes over as the comment always said it would.
chelsealong
82e6c46b94