X1b track view thumb does not work

In track view, in the bottom right hand corner, there is a triangle with dots (called a Thumb), with which you can resize a window.
 
If I put my mouse over it, I get the double-headed angled cursor.
 
But when I press the mouse button down and try resize the window, nothing happens.
 
And when I then let go the mouse button, the thumb has disappeared!
 
What is this all about? A bug? I use XP sp3.