procedure TForm1.Timer1Timer(Sender: TObject);
var
  pt: TPoint;
  h: HWND;
begin
  GetCursorPos( pt );
  if PtInRect( Self.BoundsRect, pt ) then
    exit; // Over own window

  h := Winapi.Windows.WindowFromPoint( pt ); // Handle of control under the mouse cursor
  if ImageEnView1.IO.CaptureFromScreen( iecsSpecifiedWindow, -1, h ) = False then
    ImageEnView1.IO.CaptureFromScreen( iecsSpecifiedWindow2, -1, h );
end;

...
private
  { Private declarations } 
  ...   
  FCurrentHash: AnsiString;
  FOldHash: AnsiString;
  ...

GetCursorPos(FPoint);
if PTInRect(Self.BoundsRect, FPoint) then EXIT; // do not capture this window

FHandleOfWindowAtMousePos := Winapi.Windows.WindowFromPoint(FPoint); // Handle of Control under the mouse cursor
if FHandleOfWindowAtMousePos = FOldHandle then EXIT; // block the same handle as before
//CodeSite.Send('TForm1.Timer1Timer: FHandleOfWindowAtMousePos', FHandleOfWindowAtMousePos);
FOldHandle := FHandleOfWindowAtMousePos;

ImageEnView1.IO.CaptureFromScreen(iecsSpecifiedWindow, -1, FHandleOfWindowAtMousePos);
FCurrentHash := ImageEnView1.IEBitmap.GetHash();
if FCurrentHash = FOldHash then // if current bitmap is the same as before
begin
  ImageEnView1.IO.CaptureFromScreen(iecsSpecifiedWindow2, -1, FHandleOfWindowAtMousePos); // alternative capture method
  //CodeSite.Send('TForm1.Timer1Timer: alternative capture method');
end;
FOldHash := ImageEnView1.IEBitmap.GetHash();

ImageEnView1.Update;