I don't exactly get why you added the new markup because we alredy have the `thread-messages` container that could use the flex layout? Or is it because the header needs to be separated because it's handled in building blocks?

It makes also probably sense to put all the 'absolute' panels in the end of the `thread-messages` element: information-report, information-participant, edit form. Edit form is already at the end but not the others.