May the state of modifier keys influence MouseEvent.button?

[jfirebaugh]

Consider:

window.document.addEventListener('mousedown', e => console.log(e.button));

On the Mac, with default system settings for modifier keys, depressing the primary mouse button with the control key simultaneously depressed produces output that varies between browsers:

• Chrome and Safari produce 0
• Firefox produces 2

Does the spec have an opinion on which behavior is correct, or is this left as unspecified platform-dependent/browser-dependent behavior?

[garykac]

The spec does not currently have an opinion about behaviors where different browsers have some historical implementation differences. In general, we're working toward having a more algorithmic approach to specs (which will prevent these differences in the future), but UIEvents is an old spec that covers features with a lot of baggage that will be challenging to clean up.

However, we do want to (a) document the differences better, and if possible, (b) come to general agreement on what the behavior should be.

Thanks for the comment. I'm leaving this bug open to track updating the spec to provide a better description of how different browsers handle modifier keys for mouse clicks.