TControl.WMLButtonUp的inherited的作用——是为了给子类控件新的处理消息的机会

procedure TControl.WMLButtonUp(var Message: TWMLButtonUp);
begin
inherited; // 注意，如果是直接点击Form1，会执行TCustomForm.DefaultHandler(var Message);相当于给子类控件提供了新的处理消息的机会
if csCaptureMouse in ControlStyle then MouseCapture := False;
if csClicked in ControlState then
begin
Exclude(FControlState, csClicked);
if PtInRect(ClientRect, SmallPointToPoint(Message.Pos)) then
Click;
end;
DoMouseUp(Message, mbLeft);
end;