In the 15 or so years I've been using Mac's I've never run across this odd behavior. I have no idea what I did in Mac Mail preceding this behavior. I have six or seven accounts in my mac mail. With one account - and it is only in one account - when I click on the "Compose new message" icon the new message field opens and extends far beyond the edge of my monitor. I have attached a couple of photos to illustrate. I have dual monitors, and the field stretches just beyond both cinema displays.