Windows的自带控件（比如TButton）大多数消息都由它自己处理，Delphi覆盖了那么多WM_函数优先级较低，一般用不上

procedure TWinControl.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  with ThemeServices do // 这里下断点
  if ThemesEnabled and Assigned(Parent) and (csParentBackground in FControlStyle) then
    begin
      { Get the parent to draw its background into the control‘s background. }
      DrawParentBackground(Handle, Message.DC, nil, False);
    end
    else
    begin
      { Only erase background if we‘re not doublebuffering or painting to memory. }
      if not FDoubleBuffered or
         (TMessage(Message).wParam = TMessage(Message).lParam) then
        FillRect(Message.DC, ClientRect, FBrush.Handle);
    end;

  Message.Result := 1;
end;