OnMouseDown event dispatcher.
procedure MouseDown(Button: TMouseButton; Shift: TShiftState; X: Integer; Y: Integer); virtual;
Override the protected MouseDown method to provide other responses in addition to calling the OnMouseDown event handler when the user presses mouse button while the cursor's hotspot is over the managed control.
A designer calls MouseDown in response to any of the Windows mouse-down messages (WM_LBUTTONDOWN, WM_MBUTTONDOWN, WM_RBUTTONDOWN) 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.