I think you mean double-clicking on the title bar because double clicking on the system menu closes the form.
WM_SYSCOMMAND should work since the sequence of messages when double-clicking on the title bar to restore the form is:
Message posted: hwnd=$004E0820 WM_NCLBUTTONDBLCLK wParam $00000002 lParam $000705D4 Process Project1.exe (2380)
=> Message sent: hwnd=$004E0820 WM_SYSCOMMAND restore cmd requested (-44,-44) Process Project1.exe (2380)
Message sent: hwnd=$004E0820 WM_WINDOWPOSCHANGING wParam $00000000 lParam $0012F4CC Process Project1.exe (2380)
Message sent: hwnd=$004E0820 WM_GETMINMAXINFO wParam $00000000 lParam $0012EF6C Process Project1.exe (2380)
Message sent: hwnd=$004E0820 WM_NCCALCSIZE wParam $00000001 lParam $0012F4A0 Process Project1.exe (2380)
Message sent: hwnd=$004E0820 WM_NCPAINT update region 40040F4B Process Project1.exe (2380)
Message sent: hwnd=$004E0820 WM_ERASEBKGND wParam $31011DCA lParam $00000000 Process Project1.exe (2380)
Message sent: hwnd=$004E0820 WM_WINDOWPOSCHANGED wParam $00000000 lParam $0012F4CC Process Project1.exe (2380)
The problem is that the CmdType const SC_RESTORE2 = 61730 //0xF122 is missing in Windows.pas.
See the working code below:
type
TForm7 = class(TForm)
private
procedure WMSysCommand(var Message: TWMSysCommand); message WM_SYSCOMMAND;
end;
var
Form7: TForm7;
implementation
{$R *.dfm}
{ TForm7 }
const
SC_RESTORE2 = 61730; //0xF122
procedure TForm7.WMSysCommand(var Message: TWMSysCommand);
begin
case Message.CmdType of
SC_RESTORE2 : beep;
end;
inherited;
end;
Update: reference to SC_RESTORE2
from WM_SYSCOMMAND Notification on MSDN (see the "values in C#" part)