click on the small + beside the mail subject in the preview and tab panes?

I am afraid I do not see any '+'. I display the folder pane on left; email subject, from, date columns in center; and preview pane on right.

I can change the columns displayed in the center panel with the drop-down menu (perhaps that was the '+' you referred to?). But the header information does not display in the preview pane or in the opened message unless I open the message as a new window.

this one

Using that tab, I can control what headers display in the main mail pane. but in the preview pane and when I click on an individual message, no header information at all is displayed. In your screen shot, e.g., the main mail tab shows the received time of the emails; mine does as well, but when I click on the first email, say, neither the receipt time, nor any other header information is shown, just the body of the message.

I have checked with JS to see if 'header-view-toolbar' is present in the opened tab and it is; however, I cannot find that toolbar with DOM Inspector. Is there a way to force 'header-view-toolbar' to display in a tab?