Comment 8 for bug 568163

Yup, this happens on my Thinkpad T61p running 10.10 x86-64 as well:
* Left-click the applet icon and then try to adjust brightness and slider control is unclickable (and hops to the far left of the screen)
* Left-click and then right-click the icon, then right-click (to drop down the menu) and then right-click again (to clear the menu) and the control works fine.
* The applet is fully controllable with scroll keys, though it is a bit "laggy"
* Brightness is controllable as expected with the hardware keys