Add-in failing to load in OWA/New Outlook in Office attachment preview window

[krzysztof-plutoflume]

Event-driven Outlook add-ins fail to load when the user composes a reply using a standalone Word/Excel/etc attachment preview window inside Outlook on the web and New Outlook for Windows.

What's particularly unfortunate is that if there's an add-in that specifies an on-send handler (ItemSend), then the failure to load the add-in prevents the user from being able to send the email.

Your Environment

• Platform [PC desktop, Mac, iOS, Office on the web]: Windows desktop, also Office on the web
• Host [Excel, Word, PowerPoint, etc.]: Outlook
• Office version number: Microsoft Outlook Version 1.2024.911.200, Client Version 20240913004.12, WebView2 Version 128.0.2739.79
• Operating System: Windows 11
• Browser (if using Office on the web): Google Chrome

Expected behavior

Add-ins load correctly when composing a message from the attachment preview window.

Current behavior

Add-ins fail to load when composing a message from the attachment preview window.

Steps to reproduce

1. In New Outlook or OWA, find an email in the inbox that contains a Word attachment.
2. Click on the Word attachment to open an attachment preview overlay.
3. In the top-right of the overlay, click "Open in new window" to pop the attachment preview out into a new window.
4. Click "Reply" to start composing a reply.
5. If there was an OnMessageCompose add-in handler registered, it won't execute. If there was an ItemSend add-in handler registered, sending of the email will fail.

Link to live example(s)

Provide additional details

Screenshot of the relevant window in New Outlook for Windows:

Context

The issue prevents users of our on-send add-in from sending emails when using the described workflow.

Useful logs

I didn't find any obvious error logs in the developer tools console.

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