title: Layers issue using Z-Index
body: <p>I've 2 controls one on top of another: border over slider. on border user can mark a segment to appear in different color on slider.
  Because the border is written after slider in xaml, it appear on top of the slider. and that's ok.
  My problem is, the Thumb of slider also appears under the border.</p>
  
  <p>How can I set the Thumb element (belong to slider control and inside it) to appear on top of all, and also the border will appear on top of the slider?
  I tried use zIndex but without success. Any idea?</p>