This is fascinating, because the naive English interpretation of the code and the comment on that WebKit bug don't match the actual structure of the code. Here's the relevant code:

    const isInvokedByMouse = event => event.screenX > 0 || event.screenY > 0;
    const isInvokedByKeyboard = event => isEnterKey(event) || isSpaceKey(event);

Ignoring the actual conditions entirely, this code seems to be trying to categorize the event into one of two categories: mouse or keyboard. But what it actually does is to categorize into one of four categories: (mouse and not keyboard), (keyboard and not mouse), (keyboard and mouse), and (neither keyboard nor mouse). And, as the original bug shows, (neither keyboard nor mouse) is handled inappropriately. One might wonder whether (keyboard and mouse) works well.

Either the code should be deliberate about the fact that (is it a keyboard) and (is it a mouse) are separate booleans, or the code should be structured so that the actual categories are mutually exclusive. For example:

    const isInvokedByMouse = ...

and use !isInvokedByMouse to check for keyboardiness, or:

    const eventSource = ... (returns "keyboard" or "mouse")

or, perhaps even better:

    const eventSource = ... (returns "keyboard", "mouse", or "not sure")