Comment 182 for bug 403135

How to make the tray icon transparent depends on the theme. If the tray icon area uses a uniform background color, then the tray icon background can simply be filled with the same color. That is what happens with "old" desktop themes, and works in pretty much any application.

However, if the tray icon area background is _not_ uniform, then composition is required, so that the icons are really transparent. To that end, applications must create an X window for they tray icon, that uses the ARGB X11 visual. Unfortunately, some applications hence fail to do that. Instead, they use the old method and fill the background of their tray icon with an uniform background color.

I know at least VLC and Skype fall in this category. In both cases, ARGB visuals are disabled due to complications with video rendering, namely the XVideo extension cannot be used with ARGB visuals. In principle, this is not an issue: XVideo is used for the main application video rendering window, not for the tray icon window. X11 allows an application to use different visuals for different windows. However, to my knowledge, the Qt4 GUI library does not expose this feature of X11, and hence ARGB has to be disabled not just for the video widget, but for all widgets including the tray icon.