Why does the cursor disappear in the window under the track?

Is that normal? I don't think so. I have been experimenting with some different themes and I've noticed that with some themes when I move my mouse over to the pane where the track is that large area under the track itself, instead of seeing the I pointer, there is nothing.

Does that mean there is something wrong with the theme?

Why does the cursor disappear in the window under the track?