The emacs mode (which you can turn on via your account settings page) comes from an open source project called codemirror. It's evolving very quickly, so it's missing a few things, and there aren't any official docs for it yet.
To see what keys are supported, the only option at the moment is to look at the keyMap variable in the source here: http://codemirror.net/keymap/emacs.js
Browsers often use key combinations used by Emacs, including Ctrl-N, Ctrl-T and Ctrl-W, to open or close windows or tabs. These browsers intercept the events before they are sent to Overleaf, so unfortunately there is nothing we can do to change this.
Depending on your browser and operating system, you may be able to work around this problem by remapping your browser's keyboard shortcuts. For example, there is a FireFox extension (and that is not the only one) that will let you remap those key combinations so that they no longer trigger actions in the browser.
It is also worth noting that the problems with Ctrl shortcuts do not affect browsers on Mac OS, which generally use the command key for those functions.