Problem:
SOGo renders a non-root text/html MIME part with

Content-Disposition: inline

inside a multipart/related message as additional visible message body content.

The same message is displayed correctly in Thunderbird.

Minimal MIME structure:

+-- text/html <-- related non-root part
Content-ID: <test-html-resource>
Content-Disposition: inline

There is no "start" parameter on the outer multipart/related, therefore the first body part (multipart/alternative) is the root object.

Actual behavior in SOGo:
SOGo displays both

the normal HTML message body, and
the additional related text/html part below the message.

If the related HTML part contains CSS such as position, z-index or large backgrounds, this can severely break the message display.

Expected behavior:
Only the root object of multipart/related should be displayed as the message body.

Non-root related MIME parts should not automatically be rendered as additional message body content.

Thunderbird behavior:
Thunderbird displays only the actual root HTML message. The additional inline text/html related part is not appended to the visible message body.