Right button is button3 rather than button2. Accept either and treat
both as "not left" to pass CLICK_2 back to the core.
Treat <Ctrl>+left-click as "not left" too, to simplify usage with Mac
one-button mouse (where <Ctrl>+left-click can be configured to send
"secondary click" but might not be set do so) or one-button laptop
trackpad (where having two fingers on the scrolling portion of the pad
while clicking the button is necessary to send "secondary click").
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