Comment by mike_hearn

That's the issue you'd face with today's operating systems and UI designs, yes, but I'm talking about a hypothetical new OS design.

In such an OS the APIs would allow you to atomically transfer focus as part of other operations, for example, starting a new program or opening a new window could simply transfer focus atomically to the pending new program/window such that the OS buffers keystrokes until the recipient is ready to receive them. Also, taking focus would require you to advertise what keys you're willing to receive, allowing a focus cascade such that there's never any situation in which keystrokes get delivered to a UI that isn't able to do anything with them. At the top level of the shell there'd be a kind of global command line or palette that is the default receiver of all other unhandled keystrokes. Because focus transfer is always deterministic under this scheme, people can learn where keystrokes will go without timing playing a part.