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вроде уже выкладывал по этой теме:

Код:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
  private
    { Private declarations }
  public
    { Public declarations }
    procedure WndProc(var Message: TMessage); override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TForm1 }

procedure TForm1.WndProc(var Message: TMessage);
const
  n = 4;
var
  x, y: Integer;
begin
  case Message.Msg of
    WM_NCHITTEST: begin
      x:=LOWORD(Message.LParam);
      y:=HIWORD(Message.LParam);
      if (Abs(Left+Width-x)<n) and (Abs(Top+Height-y)<n) then Message.Result:=HTBOTTOMRIGHT
      else if (Abs(Left-x)<n) and (Abs(Top+Height-y)<n) then Message.Result:=HTBOTTOMLEFT
      else if (Abs(Left+Width-x)<n) and (Abs(Top-y)<n) then Message.Result:=HTTOPRIGHT
      else if (Abs(Left-x)<n) and (Abs(Top-y)<n) then Message.Result:=HTTOPLEFT
      else if Abs(Left-x)<n then Message.Result:=HTLEFT
      else if Abs(Top-y)<n then Message.Result:=HTTOP
      else if Abs(Left+Width-x)<n then Message.Result:=HTRIGHT
      else if Abs(Top+Height-y)<n then Message.Result:=HTBOTTOM
      // опционально
      else Message.Result:=HTCAPTION; // можно таскать за клиентскую область
//      else Message.Result:=HTCLIENT; // нельзя таскать за клиентскую область
      Exit;
    end;
  end;
  inherited WndProc(Message);
end;

end.