Thank you for your answer.
I've solved it. The problem was the JPanel. I put the JList directly in the JScrollPane, and then I have added the scroll into the panel. Now the panel(not the scroll) is nested by the JSplitPane. And it works like a charm. The main idea is that the JList must be placed directly in the JScrollPane, for the lather to take notice of JList's method "boolean getScrollableTracksViewportWidth()"(Scrollable interface), which returns true in the case of horizontal wrapping. JPanel doesn't implement Scrollable, unfortunately.
For specific details, this is a proper source:
Scrollable tips.