If you go into WormNET and click on the Chat history which disables the Chat input box along the bottom of the screen then minimize. When you return to the game clicking on the Chat Input box will not activate it, at least not until you click on another element like the Chat history, a game name or the Player List.

Windows 7 x64, W:A 3.6.30'


edit: occasionally when it is active while minimizing, when you return you can't type until you click somewhere else first then activate the chat again.