[mds]

In a rational environment if you're leaving "for cause", it shouldn't come as a shock to your employer when you quit, in the same way a bad performance review or firing shouldn't come as a shock to the employee. There should be feedback in both directions, if they're not meeting your career goals or you're not meeting their performance goals.

But as a rule I wouldn't tell my boss I'm actively interviewing until I have an offer in hand. Just like, and for a lot of the same reasons, the company as a rule isn't going to inform under-performing employees that they're actively looking to hire someone to replace them before firing them.