You would need a lot of real-world analysis to make this work right. Consider the case of 10 annoying dialog boxes that pop up in sequence, which the user press

You would need a lot of real-world analysis to make this work right. Consider the case of 10 annoying dialog boxes that pop up in sequence, which the user presses enter to close. The no-focus-stealing OS thinks the user can't possibly respond to a new window in less than 180 ms, and redirects input to some irrelevant parent window. Yet in this case, the user is already in a beat pattern with the annoying dialog boxes, and is responding in < 50 ms. There are probably a bunch of other special cases that are hard to imagine unless you're a full-time window-manager developer (I'm not.)