JS mouse_caps incorrect value

If both L/R mouse buttons are down, releasing either button sets mouse_caps to zero instead of reflecting the actual mouse state.

Action: Press both buttons then release the right button.
Expected: mouse_caps is 1 (left button still pressed)
Result: mouse_caps is 0

Action: Press both buttons then release the left button.
Expected: mouse_caps is 2 (right button still pressed)
Result: mouse_caps is 0

Action: Press both buttons and CTRL, then release the left button.
Expected: mouse_caps is 6 (ctrl + right button still pressed)
Result: mouse_caps is 0 - No longer the case