MouseUp Event not firing

[MSDN]

Hello,

On the client side in JavaScript my onMouseUp event is NOT firing??

The onMouseDown and onMouseUp events fire if there is no other event in
between.

This is what I mean:
The onMouseUp event does NOT fire after the onMouseDown followed by moving
the mouse then releasing it.

Basically I Press the Mouse button down drag a pixel in any direction then
release the mouse button will not cause the OnMouseUp event to fire.

Why??

Does the mouse position has to be in the exact location for the onMouseUp
event to fire?

Thanks

SA