KDE 4 has an effect that "dims" the parent window when a child/dialog window is opened, its found under "System Settings/Desktop Effects/All Effects/Focus" and described as: "darkens the parent window of the currently active dialog".
As far as I know its enabled by default (but I could be wrong).
If you make a simple program under Lazarus, 1 form, with 1 button and a SelectDirectoryDialog and on click of the button do: SelectDirectoryDialog1.Execute; you will see that after closing the Directory Dialog the main form will remain darkened. Minimizing/Maximizing solved the issue.
I searched these forms for anything KDE related but did not find this issue yet or a solution for it. Since it works okay with other programs I sort of feel its a lazarus specific issue ?
-Cal