The TMouseDownEvent type is a common event type that is used by controls to provide notification that a mouse button is being pressed.
The Sender parameter represents the class instance that triggered the event. The Button parameter represents the ordinal button code of the mouse button pressed, and the ShiftKey, CtrlKey, AltKey parameters represent whether the Shift, Control, and/or Alt keys were also pressed. The X and Y parameters indicate the horizontal and vertical position of the mouse pointer, in pixels, relative to the bounds of the control that triggered the event.
The ordinal mouse button values are defined in the WebUI unit, and are as follows: