Not sure, but I think there is a bug with $Mouse.x $Mouse.y or the documentation is misleading.
The .x/.y, .mx/.my, and .dx/.dy properties return the x and y position of the mouse relative to the active window, the main mIRC window, and the desktop respectively.
Now, I created a @picwin, and start timer to track mouse
window -pd @Test -1 -1 200 200
.timer@Test 25 1 echo -s $!mouse.x $!mouse.y
With @Test being active:
Hovering the mouse inside the window returns the mouse.x/.y position relative to the window
Move the mouse outside of @Test, and it returns the mouse.x/.y position relative to the window it is currently over.