Drag and drop of mails not working after initial navigation to Roundcube Web Mail #61

Drag and drop of mails does not work correctly when Roundcube is accessed through Nextcloud.

Steps to reproduce

1. In Nextcloud, click on "Roundcube Web Mail"
2. Try to drag and drop a mail → does not work
3. Navigate to Contacts or Settings within Roundcube
4. Return to "E-Mail"
5. Try to drag and drop a mail → works as expected

Expected behavior

Drag and drop should work immediately after opening Roundcube Web Mail, without needing to navigate away and back.

Environment

• Nextcloud version: 32.0.5 and 32.0.6
• mail_roundcube version: 1.2.2
• Nextcloud domain: nc.my.domain
• Roundcube domain: rc.nc.my.domain
• Hosting: Self-hosted Nextcloud on bare metal, Roundcube runs in Docker
• Tested browsers: Brave, Chrome, Firefox

Apache2 proxy configuration

<Location /roundcube>
    ProxyPass https://rc.nc.my.domain/
    ProxyPassReverse https://rc.nc.my.domain/
    ProxyPreserveHost Off
</Location>

Additional information

• Accessing Roundcube directly under rc.my.domain works without problems.
• No errors found in Nextcloud logs or the browser console.