Hi, there is a problem with popups things when caller component is inside a panelGroup with scrollable bars. The popup should appear outside the panelGroup over all. This only happen in IE7, Opera. Take a look the attached screenshoot:
have you tried to poke around with the css z-index property? this should raise the popup above other things. if you did that, also search google for internet explorer z-index bug, something is/was wrong with z-index on ie.