Why does the slider position of my JSplitpane change to zero and why does my JTree expand when I change the look&feel of the underlying Component?
If so as far as the SplitPane is concerned it seems that the component in the right(bottom) pane seems to have a very large getMinimumSize().width (getMinimumSize().height).
Another possibility is that the component in the left (or top) pane seems to have zero getPreferredSize().width (getPreferredSize().height). In general the SplitPane tries to honour the getMinimumSize(), getPreferredSize() and getMaximumSize() for each of the components. Therefore make sure that the components in left (top) and right (bottom) pane return good values for each of these calls.