Open up the Options menu and go to the last tab which should be HUD. Just above the Enemies section is "Hide tray tooltips during combat".
Unfortunately they still pop up now and again. The delay slider on the basic tab only goes up to 5 seconds and typing in a higher number will only have it change to a 5 when you click ok.
I have mine set to 3 seconds...and it's a loooonnnggggg 3 seconds before the system pops that tooltip for the power. If you are lettin that cursor camp on a power icon for that to pop then you are the most relaxed non twitch player I know. Set it to 5 and you should have no problems
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Unfortunately they still pop up now and again. The delay slider on the basic tab only goes up to 5 seconds and typing in a higher number will only have it change to a 5 when you click ok.