Fourth button, typically the Browser Back button.
Fifth button, typically the Browser Forward button.
Main button pressed, usually the left button or the un-initialized state.
Auxiliary button pressed, usually the wheel button or the middle button (if present).
Secondary button pressed, usually the right button.
Auxiliary button pressed, usually the wheel button or the middle button (if present).
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The list of buttons that can exist.
https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/button#return_value.