Error from assigning to readonly KeyboardEvent.which

[mminer]

I've logged a few reports of this line: https://github.com/ccampbell/mousetrap/blob/master/mousetrap.js#L721

Throwing this error: TypeError: Cannot assign to read only property 'which' of object '#<KeyboardEvent>'

I can't reproduce it myself and I'm unsure which key was pressed to make that code path execute. Happened on Chrome 79 on both macOS and Windows.