Problem with email preference box

I have MacBook Pro (Retina, 15-inch, Mid 2014) and have just downloaded 10.13.1 ( High Sierra)

Everything seems to work except my email preference box no longer has a green button to expand the page. It has the normal red and amber buttons.

The pop up box is now c 36 inches wide and to action anything (eg in the junk tab) I have to keep sliding the box over to the left until eventually the action buttons appear on the right side of the box

I cannot change the size of the box from any side or corner but it is is fully moveable around the screen

I have rebooted twice

Any suggestions please