Ugh.

Adding a little to your code, I tried treating both case $201 and $203 equally and this is what I came up with. It seems to work the same. Am I missing something?

mywindow=CreateWindow("Mouse Test",20,20,600,400)
mycanvas=CreateCanvas(0,0,600,400,mywindow)
SetBuffer CanvasBuffer(mycanvas)
quit%=False
Repeat
Select WaitEvent()

Case $203 ; mouse move
mx=EventX()
my=EventY()

Cls
Text EventX()+10,EventY(),"\Case $203 mouse move/ mx= "+mx +" my= "+my
Text EventX()+10,EventY()+18,"\Case $203 mouse move/ EventX= "+EventX +" EventY= "+EventY

Text mx+10,my+36,"\Case $203 mouse move/ mx= "+mx +" my= "+my
Text mx+10,my+54,"\Case $203 mouse move/ EventX= "+EventX +" EventY= "+EventY

Case $201 ; mouse down
mx=EventX()
my=EventY()

Cls
Text EventX()+10,EventY(),"/Case $201 mouse down\ mx= "+mx +" my= "+my
Text EventX()+10,EventY()+18,"/Case $201 mouse down\ EventX= "+EventX +" EventY= "+EventY

Text mx+10,my+36,"/Case $201 mouse down\ mx= "+mx +" my= "+my
Text mx+10,my+54,"/Case $201 mouse down\ EventX= "+EventX +" EventY= "+EventY

Case $803 ;user closed the window
quit=True
End Select
FlipCanvas mycanvas
Until quit
End