Created attachment 219607 [details]
screen shot

I do not know how much this is webkitgtk's issue and how much evolution's, but when I use webkitgtk 1.9.5 and select text/plain message with one attachment, then I get this view (see attached screenshot). The top half is just after I selected the message, notice basically no space for text of the email. The second half is how it should look like, which is properly computed when I change window's width. If I use message which is text/plain without attachment, then it is shown properly on the first show.