(Svetlina provided this article as a suggestion already).
However, when the control that is invisible is a RadControl (in this case a PageView), when that RadControl becomes visible, it notifies its child RadControls (in this case the RadSlider). That is why you should not have to apply the workaround in order to get the RadSlider to display correctly. I suppose there is something else in the project with which you were able to reproduce the problem, apart from the RadControls, that leads to the described result. In general, you should find the code that makes the RadSlider visible (or a parent of the RadSlider) and after this code is executed, call “repaint” (or “redraw”) method.
One additional thing – please test your project with the latest version of the suite, version number 2008_1_619, to see if this makes a difference.
In case this information does not help, we will really need a test project, demonstrating the problem.
the Telerik team