Hi Christofer Hagstedt,
I have seen a problem that resembles the one in the screenshot. It manifestes in one particular scenario - when you do not have a RadSlider control initially on your page and you load a slider control with AJAX. The problem in this case is that the client code for the slider is executed before the CSS files of the slider are loaded. This happens because the page is updated synchronously when it is loaded for the first time and after postback and asynchronously, after AJAX request. That is why there is no way to ensure that the skin of the slider will be loaded by the time its client-side code is executed.
You might ask why the slider needs its skin files in order to render correctly - the reason for that is because the slider creates its HTML and calculates the size of its elements on the client. Some sizes - for example the size for the increase/decrease handles are set in the CSS of the skin of the slider. In case those handles do not have their correct size by the time the slider tries to calculate the size of its track element, for example, the calculated size is incorrect.
We will try to fix this in the code of the RadSlider for a future release.
the Telerik team