If you set a message size limit, then emails over that size have a link to get the remainder. When you click this link from preview pane, Moz/TB starts downloading the email. This is a very nice feature on slow or charge-per-MB links. During this time, if you change to look at another mail in preview pane, the download is interrupted. I suggest that the download should be entirely independent once started, so you can continue to read other mail while it happens.