System.Window.Input.Key.System

[trebahl]

I found out experimentally that when left alt is pressed, the KeyEventArgs received by controls has Key == Key.System, not Key.LeftAlt, while the SystemKey property is == Key.LeftAlt. This seems gratuitously convoluted to me. What is the point of this? The documentation is mostly silent on this.

[miloush]

It corresponds to WM_SYSKEYDOWN messages. The system will handle them as needed.