[whizzter]

Chrome/FF in this case has an UI thread that becomes unresponsive for whatever reason and doesn't empty it's message-queue (that contains the mouse/keyboard/etc events).

In the case of IntelliJ there is probably some of their code accepting the event and then probably has some ad-hoc message passing by setting a field that is polled by the receiver thread under the assumption that the receiver always runs faster than a human can act, however as soon as that assumption is broken by some pause you get what you describe.