1) Error:
When using the right click in Mac, the order of events are:
-MouseDown
-ContextClick
-MouseUp

but in Windows, they are:
-MouseDown
-MouseUp
-ContextClick

To me the Mac behaviour seems correct. Also, a byproduct of the Windows behaviour is that if you catch MouseUp and call Use(), ContextClick will not be sent. This also happens on 4.5.x

2) How can we reproduce it using the example you attached.
- Open Custom/Open window
- Right click inside of it, in both Mac OS x and Windows
- Uncomment the Use() and see how in Windows ContextClick never gets sent