The TMouseWheelEvent type is a common event type that is used by controls to provide notification that the mouse wheel is being rotated.
The Sender parameter represents the class instance that triggered the event. The WheelDelta parameter represents the amount, in pixels, that the mouse wheel rotation represents, 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 WheelDelta parameter can be positive or negative, depending upon the configuration of the mouse and the direction in which the mouse wheel was rotated.