So I have Apple's mail.app in the foreground and Safari in the background.
I allow the cursor to hover over the tool bar in Safari and the item under the cursor highlights.
This is cross application suck.

Mail sucks by itself.
I move the mouse causing the cursor to move so that it hovers over an email in my spool and a little yellow window pops up telling me stuff I don't need to know which also eats up my processor cycles. If I want to know more about the underlying email I can click on it and open it. As it is it delays me as it takes processor cycles for mail to get this annoying popup out of my face so I can go on to do something else

As a consequence of posting this page to #macintosh on EFNet I found out that those annoying yellow boxes are hated by many and they are called tooltips and they can be turned off in Safari and I did so. I have not been able to locate knowledge about how to do so with mail.app
Finder-Mail-Suckage
Safari-Finder-Suckage
iPhoto-Mail-Safari-Finder-Suckage