It appears the custom hit testing logic of WidgetComponent handles click positions well via UWidgetComponent::GetHitWidgetPath adding virtual mouse coordinates to the arranged widget, but mouse movements aren't corrected in the same way. This can be remedied by disabling Slate's Fast Widget Paths, which causes FWeakWidgetPath::ToWidgetPath to call into WidgetComponent's TranslateMouseCoordinateFor3DChild to get the proper cursor position, but this causes mouse clicks to offset in the opposite direction. Ideally both clicks and movements would report the same cursor positions, so widgets like the slider can function properly in this scenario.