I have a number of machines under my bench, but (for reasons of space) normally only one screen, keyboard, and mouse on the main system ... then I VNC into the machine I want to maintain.
I needed to run a tool from my trusty PortableApps USB stick, so I plugged the stick into the machine of interest, then VNC'ed into it.
Click on the icon in the task bar - it didnt "pop-up"! Consistently!
Plug a "real" screen into that machine and click on the icon via VNC and it is visible on the real screen but not on the VNC session.
It would appear that the PortableApps application is invisible if viewing the system via VNC. The apps themselves are OK ... it's just the pop-up menu.
It did occur to me that it may be because I suppress "WALLPAPER" when connecting via VNC - but turning that off made no difference.
A forum search didnt reveal anything relevant. Running XP Pro + SP3 -- PortableApps v2.0 Beta 4