SYSTEM.MOUSE_BUTTON_MODIFIERS
SYSTEM.MOUSE_BUTTON_MODIFIERS indicates the keys that were pressed during the click, such as SHIFT, ALT, or CONTROL. The value is always a character string.
For example, if the operator holds down the control and shift keys while pressing the mouse button, SYSTEM.MOUSE_BUTTON_MODIFIERS contains the value "Shift+Control+".
The values returned by this variable will be invariant across all platforms, and will not change across languages. SYSTEM.MOUSE_BUTTON_MODIFIERS should be used in place of SYSTEM.MOUSE_BUTTON_SHIFT_STATE.
Possible values are: "Shift+", "Caps Lock+", "Control+", "Alt+", "Command+", "Super+", and "Hyper+".