Java Console is written in Swing using JFrame and JTextArea in Java Plug-in. When running Merlin with Java plug-in, the console doesn't not response to any keyboard input through the JTextArea, even various listeners have been registered, but these listeners never get called.

Notice that the same plug-in runs in Kestrel/Firefly/Ladybird without the same problem, so the problem is likely to be in Swing in Merlin.