Toggle reading pane between text and HTML

Normally for speed and less clutter I like to see articles in the reading pane in text mode.

BUT... when I want to look at the article some more I like to switch it to HTML display. What I would really like to be able to do is to double click on the line in the subject pane to get it to switch from text to html. Ideally if I then double click again it does the current behaviour of opening a new tab with the article.

Any idea on how to do this?

Cheers,

Harry