title: Mouse event on transparent background
body: <p>I have create several canvas with transparent background and wanna make some move event on it.</p>
  
  <p>However, I found that all mouse event (e.g. MouseLeftButtonDown) cannot function of the background of the canvas is Null or Transparent. But if I set the background to some solid color (say, red or blue), the mouse event function well.</p>
  
  <p>Why?</p>
  
  <p>Could I make the mouse event function properly with transparent background?</p>
  
  <p>Thank you!</p>