OnMouseUp event dispatcher.
procedure MouseUp(Button: TMouseButton; Shift: TShiftState; X: Integer; Y: Integer); virtual;
Override the protected MouseUp method to provide other responses in addition to calling the OnMouseUp event handler when the user releases a previously pressed mouse button while the cursor's hotspot is over the control.
A designer calls MouseUp in response to any of the Windows mouse-up messages (WM_LBUTTONUP, WM_MBUTTONUP, WM_RBUTTONUP) sent to managed controls, decoding the message parameters into the shift-key state and position, which it passes in the Shift, X, and Y parameters, respectively. The value of the Button parameter indicates which mouse button was released: left, right, or middle
X,Y coordinates are relative to client origin of the root pane FRootPane.