vertical grey "No HTML Message" alongside message body

Jerome Yuzyk <[email protected]>
Newsgroups gmane.comp.kde.users.pim
Organization BRIDGE Scientific Services
Message-ID <3086568.CQgPSk9U9A@desk> 
[KMail 5.8.3 on Fedora 28]

I just updated to the latest KMail this morning and now I have a vertical "No 
HTML Message" in a gray bar alongside the message body. I don't see how to 
remove it in any menus or settings.

How do I turn it off?
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