A mouse button may be remapped to Ctrl button easily like this:
RButton::Ctrl
However the same code does not work for XButton1 or XButton2.
XButton1::Ctrl
Any way to make XButton1 work as Ctrl?
Edit: When I click the XButton on the mouse, Key History shows that the LControl was pressed and released consecutively, other letter-keys I pressed while XButton1 was pressed show up being clicked before the press/release of the XButton/Ctrl hotkey.
Turns out AutoHotKey is innocent, the culprit was the mouse I was using (Logitech M650)
It appears to send the XButton1 up/down command at once on release.
Switching to a different mouse fixed the issue and AHK works as expected.