unit MovePanForm;

interface

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

type
  TMyPanel= class(ExtCtrls.TPanel)
  protected
    procedure WMMoving(var Msg: TMessage); message WM_MOVING;
  end;

  TPanel= class(TMyPanel);

  TForm1 = class(TForm)
    MyPanel: TPanel;
    procedure MyPanelMouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
  private
  public
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TMyPanel.WMMoving(var Msg: TMessage);
var
  pr: PRect;
  tl: TPoint;
  xmax, ymax: integer;
begin
  pr:=pointer(Msg.lParam);
  tl:=Parent.ScreenToClient(pr.TopLeft);

  xmax:=Parent.ClientWidth-Self.Width;
  if tl.x<0 then begin
    pr.Left:=pr.Left - tl.x;
    pr.Right:=pr.Right - tl.x;
  end
  else if tl.x>xmax then begin
    pr.Left:=pr.Left - (tl.x-xmax);
    pr.Right:=pr.Right - (tl.x-xmax);
  end;

  ymax:=Parent.ClientHeight-Self.Height;
  if tl.y<0 then begin
    pr.Top:=pr.Top - tl.y;
    pr.Bottom:=pr.Bottom - tl.y;
  end
  else if tl.y>ymax then begin
    pr.Top:=pr.Top - (tl.y-ymax);
    pr.Bottom:=pr.Bottom - (tl.y-ymax);
  end;

  inherited;
end;

procedure TForm1.MyPanelMouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  ReleaseCapture;
  MyPanel.Perform(WM_SysCommand, $F012, 0);
end;

end.

procedure TMyPanel.WMMoving(var Msg: TMessage);
var
  pr: PRect;
  tl: TPoint;
  d: integer;
begin
  pr:=pointer(Msg.lParam);
  tl:=Parent.ScreenToClient(pr.TopLeft);

  d:=Parent.ClientWidth - Self.Width;
  if d>0 then begin
    d:=tl.x - d;
    if tl.x<0 then begin
      pr.Left:=pr.Left - tl.x;
      pr.Right:=pr.Right - tl.x;
    end
    else if d>0 then begin
      pr.Left:=pr.Left - d;
      pr.Right:=pr.Right - d;
    end;
  end;

  d:=Parent.ClientHeight - Self.Height;
  if d>0 then begin
    d:=tl.y - d;
    if tl.y<0 then begin
      pr.Top:=pr.Top - tl.y;
      pr.Bottom:=pr.Bottom - tl.y;
    end
    else if d>0 then begin
      pr.Top:=pr.Top - d;
      pr.Bottom:=pr.Bottom - d;
    end;
  end;

  inherited;
end;