via right-click on any window title bar, I select "More Actions -> Configure Window Manager -> Task Switcher" There I change e.g. the checkbox for "Show selected window" and click Apply crash. Always. #4 0x00007f5387430495 in KCMKWinDecoration::borderSizeIndexToString (this=0x55edf1441380, index=-1) at /usr/src/debug/kwin5-5.18.5-1.1.x86_64/kcmkwin/kwindecoration/utils.cpp:118 #5 KCMKWinDecoration::save (this=0x55edf1441380) at /usr/src/debug/kwin5-5.18.5-1.1.x86_64/kcmkwin/kwindecoration/kcm.cpp:152 #6 0x00007f53944dc892 in KCModuleQml::save (this=0x55edf147de50) at /usr/include/c++/9/bits/unique_ptr.h:360 #7 0x00007f53944d9257 in KCModuleProxy::save (this=<optimized out>) at /usr/src/debug/kcmutils-5.70.0-1.1.x86_64/src/kcmoduleproxy.cpp:239 #8 0x00007f53944dfed8 in KCMultiDialogPrivate::apply (this=<optimized out>) at /usr/src/debug/kcmutils-5.70.0-1.1.x86_64/src/kcmultidialog.cpp:371 ...
Seems the m_borderSizeIndex variable is never changed and is still -1 when trying to save ...
*** This bug has been marked as a duplicate of bug 445136 ***