I have often thought about trying to figure out whatever Win32 API is responsible for focus stealing and neuter it down to something akin to

  void steal_focus_when_user_is_least_expecting() {
    // stub, much better :)
  }

Not just frustrating, it's a security hole. Stealing focus means a user may expect to be typing a password but find it's inputted somewhere they did not expect.

Problem it isn't an API, it is just unexpected consequences of how a few things work. Fixing this just isn't easy as the simple attempts will break even more than the frustrating thing you are trying to fix - and thus be worse.

That said, Microsoft should have fixed this long ago - it is hard but a few people can do it given a few years to work through all the special cases.