In brower when ESC pressed it only gives you your cursor back in the WebClient In Firefox/Chrome.
I tried to press esc on Chrome and it gave me my cursor back instead of opening the menu to enable fly hacks like I got to do on mobile.
if you are on chrome you have to hold esc for about 3 seconds to bring up the options menu, which was added in a recent update.
Unfortunately, browsers do not send a 'key down' event when Escape is pressed to release the mouse cursor - so the game is not even aware that Escape was pressed. There also isn't a browser API that lets me just check the current press state of keys.
Browsers do provide a 'mouse lock lost' event however - I could change the game to show the Pause menu whenever that happens. I haven't yet because I'm not sure if that would be annoying to people