You've had the option to reply to message notifications on Android for years, so why can't you do that on your shiny new Chromebook? You can soon. Google has started implementing support for in-line replies to messages from notifications. Much as on Android, you can respond to a message from a supporting app (Hangouts is one example) in the pop-up box rather than switching tasks entirely and losing your focus.
The feature isn't yet included in public builds, and it's clearly unfinished. Google itself warns that there aren't even animations and a "submit" icon to send a message with a click. It'll likely take weeks before you can easily try this yourself. When it does arrive, though, it should make your life considerably easier if you want to keep a conversation going while you're in the midst of a Netflix marathon.