Originally Posted by Woodsman
By resizing do you mean going from windowed mode to full screen and vice-versa?
In your VM do you have compositing enabled? How about other desktop effects? Have you tried disabling all desktop effects?
I'm sorry for not being more explicit. I mean resizing a VirtualBox guest window in windowed mode, with the entire KDE desktop in a single guest window. Seamless mode is disabled and full-screen is disabled. Scaled mode is also disabled. Resizing does not cause the problem, but it sometimes fixes the problem. The panel temporarily displays the correct silver color until the display is next updated. Most of the time the panel is the wrong color. The panel is usually solid black or solid white, and only sometimes silver. Switching to seamless or full-screen mode does not fix the problem. The panel stays black or white.
In the guest OS (Slackware 14 + KDE 4.10.2) I have compositing disabled using a file "/etc/X11/xorg.conf.d/10-disable-compositing.conf".
Option "Composite" "Disable"
I also have the KDE desktop effects disabled in the guest OS. I've never been able to get desktop effects to work in VirtualBox, so I don't use the experimental 3D graphics driver at all. I'm using the normal VirtualBox Guest Additions graphics driver.
I have tried deleting the KDE cache under "/var/tmp" and also creating a new user account with no previous KDE settings. Neither of those affected the problem.
The host OS running VirtualBox also does not matter. It happens with a Windows 7, Windows XP or Slackware + KDE host OS.
I haven't tried any other Linux guest operating systems or Linux desktop environments. Windows 7 and Windows XP running as a guest OS do not have the display problem. Also, a Slackware 14 guest running KDE 4.9 does not have the problem.