Swipe navigation in the header area, which recently lost navigation arrows, is presumably still supposed to work regardless of message body width, but does not.
Wide message bodies take over swipe navigation such that you have to scroll to the edge of the content before it will swipe to another message. I’m not a fan … but in a previous topic the header was highlighted as a way to navigate that was unaffected by wide bodies.
This is not the case however. A new message with a wide body cannot navigate right to an old message without scrolling the body first. Also, once the body is scrolled by any amount from the left margin, swiping to the left in the header area is broken.
Given that there used to be arrows that always worked, and that the header is a different UI component that the message body, makes this a UI regression/bug IMO.