On a screen such as Inbox messages, whenever the cursor is over the subject of a message, there is a pop-up message near the cursor which says “Tip: Drag subject to a folder to move message(s)”. I understand the intention of this message, but the implementation needs improvement. Not only does the ‘Tip’ appear when the cursor is over an actual message subject, but also when the cursor is in the area where message subjects would be shown if the list were longer. It also appears in other areas of the page at times - even in the tab title!
I haven’t tried it yet with other browsers, but I suspect it is a CSS issue.