fix: prevent duplicate iframes on CldUploadWidget unmount/remount

[PaulBoye-py]

Description

When CldUploadWidget unmounts and remounts, next/script's onLoad callback fires again (the script is already cached in the browser, but the callback re-registers on remount). The existing triggerOnIdle callback had no guard against the component unmounting before the idle callback fired, thus allowing a zombie widget instance to be created with an orphaned iframe in the DOM.

Root cause: triggerOnIdle defers widget creation asynchronously via requestIdleCallback. If the component unmounts during that window, the cleanup runs (widget.current = undefined), but the pending idle callback still fires, passes the !widget.current check, and creates a new widget on an unmounted component.

Fix: Add an isMounted ref that gates both the handleOnLoad entry point and the async idle callback. The single cleanup useEffect sets isMounted.current = false before calling widget.current?.destroy(), ensuring no in-flight callbacks can recreate the widget after cleanup.

A manual test page has been added at tests/nextjs-app/app/test-widget/page.tsx to verify the fix via unmount/remount cycling in the browser.

Issue Ticket Number

Fixes #587. Supersedes #588.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have followed the contributing guidelines of this project as mentioned in CONTRIBUTING.md
• I have created an issue ticket for this PR
• I have checked to ensure there aren't other open Pull Requests for the same update/change
• I have performed a self-review of my own code
• I have run tests locally to ensure they all pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes needed to the documentation

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Project	Deployment	Actions	Updated
next-cloudinary	Ready	Preview	Sep 15, 2026 9:09pm UTC

[PaulBoye-py]

supersedes #588