FULL PRODUCT VERSION : all versions (up-to and including the mustang snapshot build 44) ADDITIONAL OS VERSION INFORMATION : irrelevant EXTRA RELEVANT SYSTEM CONFIGURATION : irrelevant A DESCRIPTION OF THE PROBLEM : The problem is any code in the ui-delegates that tries to get hold of a special type of LookAndFeel and ui-delegate class: if (mycomp.getUI() instanceof SomeUI) { dosomething(); } myUI = mycomp.getUI(); if (myUI instanceof SomeUI) { dosomething(); } myUI = BasicLF.getUIOfType(mycomp.getUI(), SomeUI.class); if (myUI != null) { dosomething(); } (similar for checking the type of LookandFeel) In all these cases the dosomething() is not executed if an auxiliary LF is added: then the getUI() returns MultiLookAndFeel and the instanceof check is false. STEPS TO FOLLOW TO REPRODUCE THE PROBLEM : nothing to reproduce - some behaviour silently isn't executed. Before fixing #4842445 an error was thrown. The fix solved the underlying reason - the returned ui is MultiLookAndFeel - only incompletely. REPRODUCIBILITY : This bug can be reproduced always. ---------- BEGIN SOURCE ---------- no code - it's a code quality error! I rate the severity high because any application is potentially unstable as soon as an aux LF is set. This instability may express itself at unexpected places ("loading audiomap doesn't work") , the real reason difficult to locate and such requiring long and difficult debugging sessions. There's nothing an application programmer can do to solve - short of not adding aux LF. ---------- END SOURCE ---------- CUSTOMER SUBMITTED WORKAROUND : no workaround, this is solvable in core swing only. All ui-delegates need to be aware of the MultiLF: ui = mycomp.getUI if (uiinstanceof MultiXXUI) ui= ((MultiXXUI) myui).getUIs()[0] myui = BasicLF.getUIOfType(ui, someui.class); if (myui != null) dosomething(); similar for LookAndFeel types. In the long run this should be moved as a public service into LookAndFeel: this requires to add some means to recognize a MultiXXUIs - f.i. an interface with the method getUIs and the same contract as the multiXXUIs getUIs(). It's important to guarantee that the first entry has the "real" UI. Have fun when cleaning up <g> Jeanette