Background tabs, timers and reduced motion

Browsers throttle hidden tabs: setTimeout/setInterval fire at most once per second (once per minute after 5 minutes in Chrome), requestAnimationFrame does not fire at all, and the whole tab may be frozen. Signals, effects and DOM updates keep working synchronously; only time-based helpers are affected:

• debounced(), throttled(), interval(), timeout(), @input.debounce.N — fire late, in order, never twice.
• poll() sleeps in a hidden tab by default ({ background: true } to keep polling); resource({ refetch: { focus } }) refetches when the tab becomes visible.
• Transitions and spring()/tween() are rAF-driven; a 40 ms timer fallback finishes the CSS contract so hidden and classes never get stuck.
• virtualScroll({ mode: 'window' }) recomputes its window on visibilitychange.
• resource({ offline: true }) yields its IndexedDB connection when another tab upgrades the database and reports a blocked upgrade instead of hanging.

prefers-reduced-motion (an OS setting) turns every transition into an instant state change (defaults.motion === 'auto'). Tests and demos should pin defaults.motion = true or false explicitly.