Every desktop environment has its share of good and bad features, and a few good features that some find a little bit annoying. The Taskbar Thumbnail is one of those good features that manages to be, for some, annoying.

If you use KDE, you probably know what I am referring to. If not, see the first screen shot below. It shows one quadrant of a desktop with the thumbnail of an open application’s window in focus. That happens when you mouseover the application’s icon on the panel. Then, if you mouseover the thumbnail, the application’s window gains focus.

The annoying part happens when that effect is produced unintentionally, as sometimes takes place in the course of moving the mouse cursor from point A to point B. If, like me, you find it annoying, this guide will show you how to disable it. And it is very simple process, all point-and-clicky.
KDE Taskbar Thumbnail

The tool that helps you get the job done, is the Desktop Effects module in KDE’s System Settings. If running ROSA Marathon or Mandriva Linux, an icon for System Settings is on the panel. For other distributions that use the Kickoff menu, you will find an entry for it in the Favorites column of that menu, or in the System menu category. When open, click on the Desktop Effects module.
KDE Control Center

On the Desktop Effects window, click on All Effects tab, then scroll to the bottom of the Appearance section. You should see Taskbar Thumbnails enabled. Disable it. Click Apply and close System Settings.
KDE Desktop Effects

Now, a mouseover action on an open application’s icon on the panel, whether intentioned or not, will only show a tooltip. While mousing over the tooltip still produces the same effect as on a thumbnail, it is less likely to occur with the tooltip because of the reduced desktop real estate it occupies.
KDE Taskbar Thumbnail Disabled