말그대로 message drain입니다.
즉 videowindow의 마우스 이벤트를 받기 원하는 원도우 핸들을
넣어주시면 됩니다.

그러면 video window에서 발생하는 마우스 메시지가
설정된 윈도우로 postmessage됩니다.

보통은 판낼 같은 것을 만들고, 메시지를 잡도록 한다음 처리하지요.
메시지를 직접 잡을지 델파이가 내부처리를 할지 모르겠군요.

참고로 저는 다음과 같이 합니다만... ^^;

        procedure TDSVideoWindow.DrainWndProc(var Msg: TMessage);
        var
        nLeft, nTop, nWidth, nHeight: integer;
                xPos, yPos: WORD;
                bRet : boolean;
        begin
          with Msg do
                case msg of
                        // Non Mouse Pos
                        WM_KEYDOWN, WM_KEYUP:
                        begin
                                PostMessage(handle, msg, wParam, lParam);
                        end;
                        // Mouse event
                        WM_LBUTTONDBLCLK, WM_LBUTTONDOWN, WM_LBUTTONUP,
                        WM_MBUTTONDBLCLK, WM_MBUTTONDOWN, WM_MBUTTONUP,
                        WM_RBUTTONDBLCLK, WM_RBUTTONDOWN, WM_RBUTTONUP,
                        WM_MOUSEMOVE :
                        begin
                                // Translate cursor position
                                xPos := LOWORD(lParam) + FRendererLeft;
                                yPos := HIWORD(lParam) + FRendererTop;

                                lParam := MakeLong(xPos, yPos);

                                SendMessage(handle, msg, wParam, lParam);
                        end;
                        // nonclient event
                        WM_NCLBUTTONDBLCLK, WM_NCLBUTTONDOWN, WM_NCLBUTTONUP,
                        WM_NCMBUTTONDBLCLK, WM_NCMBUTTONDOWN, WM_NCMBUTTONUP,
                        WM_NCRBUTTONDBLCLK, WM_NCRBUTTONDOWN, WM_NCRBUTTONUP,
                        WM_NCMOUSEMOVE, WM_MOUSEACTIVATE :
                        begin
//                                PostMessage(handle, msg, wParam, lParam);
                        end;
                else
                  Result := DefWindowProc(FWindowHandle, Msg, wParam, lParam);
          end;
        end;



  constructor TDSVideoWindow.Create(AOwner: TComponent);
  begin
        inherited Create(AOwner);
{        ControlStyle := [csAcceptsControls, csCaptureMouse, csClickEvents,
        csDoubleClicks, csReflector]; } // lks
        
        ControlStyle := [csAcceptsControls, csClickEvents,
        csDoubleClicks, csReflector]; // lks : 마우스 capture를 직접 제어해야 하므로 빼버린다.

        self.TabStop := true;
    self.Height := 120;
    self.Width  := 160;
    self.color  := $00100010;
    canvas.OnChanging := CaptureCanvas;
        canvas.OnChange   := ReleaseCanvas;

        FWindowHandle := AllocateHWnd(DrainWndProc); // lks : drain Window
  end;

  destructor TDSVideoWindow.Destroy;
  begin
        DeallocateHWnd(FWindowHandle); // LKS : dealloc drain window
        
    inherited destroy;
  end;

  procedure TDSVideoWindow.SetGraph(grf: TDSGraph);
  var
          nLeft, nTop, nWidth, nHeight: integer; // position, size: lks
  begin
        if assigned(FGraph) then FGraph.FVW := nil;
        FGraph := grf;
        if assigned(FGraph) then
        begin
          FGraph.FVW := self;
          if VideoAvailable then
          begin
                with FGraph.VideoWindow do
                begin
                  put_Owner(self.parent.Handle);
                  put_Caption(Name);
                  put_WindowStyle(FWindowInfo.dwStyle or WS_CHILD or WS_CLIPSIBLINGS);
                  put_WindowStyleEx(FWindowInfo.dwExStyle);

                  CalcVideoPosition; // lks
                  SetWindowPosition(FRendererLeft, FRendererTop, FRendererWidth, FRendererHeight); // lks
          put_MessageDrain(FWindowHandle); // lks

                  // SetWindowPosition(self.Left,self.Top,self.Width,self.Height);
          // put_MessageDrain(Self.Handle);
        end;
      end;
    end;
  end;