+++ This bug was initially created as a clone of Bug #395346 +++ kwin git master head at e69da579f, using gentoo/kde's overlay repo kde live-git packages and qt-5.11.0-rc2 (latest available in gentoo main repo). Git commit 275b7ee0f apparently fixed a crash on kwin-wayland with aurora windecos, referring to the bug I cloned for this one. But it breaks kwin_x11 in the same way it fixes kwin_wayland -- closing a window with an aurora windeco set crashes kwin_x11! Confirmed that it's aurora windecos only by trying breeze (ok), oxygen (ok) and plastik (broken, believed to be aurora). Confirmed that reverting just that commit on current master head ( e69da579f ) fixes the problem.
*** This bug has been marked as a duplicate of bug 395732 ***