This message is posted when mouse tracking is in effect and the mouse hovers.
flags indicating which virtual keys are down - see enum MouseState.
|low-order part||The x-coordinate of the cursor.|
|high-order part||The y-coordinate of the cursor.|
If the window processes this message, it should return zero.
The cursor's coordinates are relative to the origin of the client.
Hover tracking ceases upon receipt of this message.