When I set some custom css class on the row and do postback, then my custom css class will be overwritten with the activation's ActiveRowCssClass. It doesn't add it to the existing classes like it should be, it replaces them. 

When moving through grid on client side (using arrows or mouse), then it's behaving ok - it add/removes ActiveRowCssClass from the row classes by overwriting existing classes. Problem only appears when after (partial)postback such a row is active which has some custom css class, then it will be overwritten by the ActiveRowCssClass.