This happens because the ScrollViewer measures the RadSlider control with infinity and it gives it as much space as it needs. Once measured with a bigger size, the slider won't resize with a smaller size, because the scrollviewer shows its scrollbar and tells that there is no need to resize the control.
In order to resolve this you can avoid setting the HorizontalScrollBarVisibility to Auto. Or you can manually set the Width of the RadSlider in code when the window is resized.
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