Reaching for the mouse doesn't break my flow. It makes the thing I was doing invisible in the flow. Keyboard shortcuts require me to think, which makes it FEEL like I'm doing something, and that something is in the flow so it feels like I'm productive. However the mouse doesn't even enter into the flow at all, I just do the thing and get on with the real work without breaking flow.

Again, there is no universal correct answer. Sometimes the keyboard really is better. However sometimes the mouse really is better and because I'm proficient in it I don't break my flow to use it.