It looks like that due to optimizations in the paint(Graphics g) method, the
image used for the animations in JXCollapsiblePane is not entirely repainted
(not at all, sometimes), especially if there is more than one JXCollapsiblePane
in the application or if the JXCollapsiblePane is initially collapsed.
I attached a patch, which uses a JViewport instead of a BufferedImage. It works
for me, but it is not much tested and may need some adjustments.