[Bug] Toast lifecycle: onHide double-fires for one toast, and any viewport re-render silently cancels a paused auto-hide timer

[arham766]

Two related defects in the Toast timer/dismiss machinery (both verified by execution)

1. onHide double-fires. ToastViewport.removeToast fires entry.options.onHide?.(reason) whenever the entry is still in toastsRef.current — but an exiting toast stays in that list until its exit transition ends (handleExited). The dedupe guard protects only exitingIds state, after onHide already fired. Any second dismissal inside the ~150ms exit window double-fires:

• double-click on "Dismiss notification" → onHide × 2
• auto-hide timer fires, then the dismiss() function returned by toast(...) is called (e.g. effect cleanup) → onHide('auto') + onHide(...)

onHide is the documented state-sync hook (mark-as-read, re-enable submit, telemetry), and in a backgrounded tab the exit transition can stall, widening the window to seconds.

2. A paused timer is silently cancelled by any viewport re-render. Toast's timer effect depends on startTimer, which depends on onDismiss — an inline arrow re-created on every ToastViewport render (onDismiss={reason => removeToast(entry.id, reason)}). So when any other toast is added/removed/exits, every mounted toast's effect re-runs: remainingRef resets to full and startTimer() fires unconditionally — ignoring isPausedRef. Verified: hover toast A (paused) or blur the window (paused), let toast B arrive, advance time → A auto-dismisses while still hovered/blurred. The pause features exist precisely for busy multi-toast UIs, so this defeats them in the common case (and it is what widens defect 1's window in background tabs). WCAG 2.2.1-relevant: the toast expires mid-read.

Suggested fix

• removeToast: bail out before firing onHide when the toast is already exiting (mirror exitingIds in a ref).
• Toast: hold onDismiss in a ref so the timer effect depends only on [autoHideDuration, isAutoHide], and have startTimer respect isPausedRef.

Happy to send the PR.

Astryx Version

main (d9932ea)