unit Unit79;

interface

uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs;

type
 TForm79 = class(TForm)
 private
   { Private declarations }
 public
   { Public declarations }
   procedure VisibleChanging; override;
 end;

var
 Form79: TForm79;

implementation

{$R *.dfm}

procedure TForm79.VisibleChanging;
begin

end;

end.

{ Forms.pas }

procedure TCustomForm.VisibleChanging;
begin
 if (FormStyle = fsMDIChild) and Visible and (Parent = nil) then
   raise EInvalidOperation.Create(SMDIChildNotVisible);
 if (Self = Application.MainForm) and not (csDestroying in ComponentState) then
 begin
   if Visible then
   begin
     ChangeAppWindow(Handle, not Application.MainFormOnTaskBar, True);
     ChangeAppWindow(Application.Handle, Application.ShowMainForm, True);
   end
   else
   begin
     ChangeAppWindow(Application.Handle, not Application.MainFormOnTaskBar, False);
     ChangeAppWindow(Handle, Application.MainFormOnTaskBar, True);
   end;
 end;
end;