Escap key strangely captured by javascript webshell

Escap key strangely captured by javascript webshell

I am using GateOne as a WebShell on my VPS (only way to SSH to it through
the proxy of the place...), it uses javascript to emulate a shell by
capturing keystrokes.
Everything works well, yet it seems the Esc key is special : it often is
understood wrongly by the server.
Mainly when I use VIM and press Esc to quit edit mode, it does something
else. Sometime it's simply a backspace, sometimes it deletes the line,
sometimes it inserts a copy of a part of the current line at the cursor
position...
Yet, clicking again in the Firefox window (while it already had the focus)
restores the expected behavior.
So there are two questions here :
The WebShell even performs to capture Firefox shortcuts such as Ctrl + T
(new tab), so why is the Esc key so special ?
Is there a way for me to get rid of this problem ? By correcting this
capture problem, or even, as it's only a problem with VIM, an other way
than Esc to exit edit mode ?
I'm stuck to using Firefox, it seems that IE and Chrome does not handle
WebSockets correctly enough to have my webshell working through the proxy
server (and so I couldn't test this Esc with them).
Even if there is no solution to my problem, I'm quite interested by an
explanation of this behavior.