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procedure Mark(y,hi,menge:integer; ListBox:TListBox);
var pos:real;
begin
pos:=trunc(y/hi);
if pos>menge then exit;
ListBox.Selected[Round(pos)]:=true;
end;

procedure TForm1.ListBox1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
Mark(Y,Form1.ListBox1.ItemHeight,Form1.ListBox1.Items.Count-1,Form1.ListBox1);
end;					

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procedure Mark(y,hi,menge:integer; ListBox:TListBox); 
var pos:real; 
begin 
  pos:=trunc(y/hi); 
  if pos>menge then exit; 
  ListBox.ItemIndex := Round(Pos); 
end;