You see this thingie up there ↑ ? That’s the Apple Mail interface that appears when you hover over a message header in Mail’s messages pane so you can reply to the message, forward it, or delete it. Except that the thingie doesn’t always appear when you hover over the message header: sometimes Mail ignores your hovering, and you have to click outside of the Mail window and then back inside before Mail remembers what it is supposed to do.
I hate when that happens…