When I run Snoop against my application I am unable to enter any text within Snoop itself, i.e. nothing in the filter boxes above the controls or properties, and nothing in the property values. I can paste text that's in my copy buffer, and I can even do this from the Ctrl+V keyboard command, so the keyboard input is clearly being received.
I don't know if it's relevant but the application in question is a WinForms application hosting a WPF control. Against other applications I don't suffer this problem.