to reproduce, run any code having a collapsible (or taskpane), f.i. JXCollapsiblePaneVisualCheck: content not visible.
Seems to have happened on fix of
SWINGX-1444 when removing the constructors taking layoutManagers: while true that client code intention was to set the contentPane's layout, internal code might need the BorderLayout on the collapsible itself. Not entirely sure, but re-inserting that one line setting supers layout to BorderL seems to fix the issue: