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procedure TForm1.Memo1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  Panel1.SetFocus;
end;					

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// globale Variable
var oldControl:tWinControl;

//[...]
procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
    oldControl:=Form1.ActiveControl;
end;

procedure TForm1.Memo1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
    Form1.ActiveControl:=oldControl;
end;					

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unit Blah;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls;
  // Keine Ahnung welche Units überflüssig sind. Musst selber mal schauen.

type
  TMyMemo = class(TMemo)
  protected
    procedure WMSetFocus(var Message: TWMSetFocus); message WM_SETFOCUS;
  end;

implementation

procedure TMyMemo.WMSetFocus(var Message: TWMSetFocus);
begin
  Windows.SetFocus (Message.FocusedWnd);
  Message.Result := 0;
end;					

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protected
  procedure WMContextMenu(var Message: TWMContextMenu); message WM_CONTEXTMENU;
...
implementation

procedure TMyMemo.WMContextMenu(var Message: TWMContextMenu);
begin
end;