procedure TForm2.btnCountVisibleThumbsClick(Sender: TObject);
var
  i: Integer;
  VisibleThumbs: Integer;
begin
  if pgc1.ActivePageIndex = 0 then
  begin
    pgc1.ActivePageIndex := 1;
    ImageEnMView1.ViewY := 0; // show first row
    VisibleThumbs := -1;
    for i := 0 to ImageEnMView1.ImageCount - 1 do
    begin
      if not ImageEnMView1.IsVisible(i) then
      begin
        VisibleThumbs := i;
        BREAK;
      end;
      pgc1.ActivePageIndex := 0;
    end;
  end
  else
  begin
    VisibleThumbs := -1;
    for i := 0 to ImageEnMView1.ImageCount - 1 do
    begin
      if not ImageEnMView1.IsVisible(i) then
      begin
        VisibleThumbs := i;
        BREAK;
      end;
    end;
  end;
  lblVisibleThumbs.Caption := 'Visible thumbs (even partially visible): ' + IntToStr(VisibleThumbs);
end;

procedure TForm2.FormShow(Sender: TObject);
begin
  // Workaround:
  pgc1.ActivePageIndex := 1;
  pgc1.ActivePageIndex := 0;
  // Works only in FormShow (or FormActivate)!
end;

procedure TForm2.btnCountVisibleThumbsClick(Sender: TObject);
var
  i: Integer;
  VisibleThumbs: Integer;
begin
  ImageEnMView1.ViewY := 0; // show first row
  VisibleThumbs := -1;
  for i := 0 to ImageEnMView1.ImageCount - 1 do
  begin
    ImageEnMView1.UpdateImage(i);
    if not ImageEnMView1.IsVisible(i) then
    begin
      VisibleThumbs := i;
      ImageEnMView1.UpdateImage(i);
      BREAK;
    end;
  end;
  ImageEnMView1.Update;
  lblVisibleThumbs.Caption := 'Visible thumbs (even partially visible): ' + IntToStr(VisibleThumbs);
end;