client/components/tinymce/plugins/wpcom-view/plugin.js
function isSpecialKey( key ) {
		return (
			( key <= 47 &&
				key !== VK.SPACEBAR &&
				key !== VK.ENTER &&
				key !== VK.DELETE &&
				key !== VK.BACKSPACE &&
				( key < 37 || key > 40 ) ) ||
			key >= 224 || // OEM or non-printable
			( key >= 144 && key <= 150 ) || // Num Lock, Scroll Lock, OEM
			( key >= 91 && key <= 93 ) || // Windows keys
			( key >= 112 && key <= 135 )
		); // F keys
	}
function isSpecialKey( key ) {
		return (
			( key <= 47 &&
				key !== VK.SPACEBAR &&
				key !== VK.ENTER &&
				key !== VK.DELETE &&
				key !== VK.BACKSPACE &&
				( key < 37 || key > 40 ) ) ||
			key >= 224 || // OEM or non-printable
			( key >= 144 && key <= 150 ) || // Num Lock, Scroll Lock, OEM
			( key >= 91 && key <= 93 ) || // Windows keys
			( key >= 112 && key <= 135 )
		); // F keys
	}
Excludes arrow keys, delete, backspace, enter, space bar. Ref: https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent.keyCode