If you press e.g. Command+J on macOS, you get press event for both Command and J. But when you release the keys, you only get release events for Command. So J ends up stuck down from the server's perspective.
In most cases, this doesn't matter, and it will resolve itself once focus is lost or J is pressed by itself. Still, there are probably cases where it is important.
Upstream bug report:
Browser bug reports:
Workaround got added upstream:
Unfortunately, it means that applications cannot tell how long the key is pressed down. So it is not ideal.