it's a hack that people figured out on the original Apple II--you bring the shift key and tie it to the pushbutton 2 input on the paddle connector. then this became "canonized" in the Apple IIe with a motherboard jumper called X6, open by default. my IIe Platinum has this turned on by default! presumably this is the problem.
@tubetime The IIe technical reference manual (1987 version) mentions that the shift key mod was made standard on platinum machines so as to enable shift+click functionality in mouse based applications