As described in bug 388759 plasmashell gets into a repaint loop, using 100% CPU if the screen setup is changed during a session. The issue apparently has been solved with Qt 5.10.0 and following patch should be backported to Neon (if a near term upgrade to 5.10 is not feasibly): * 6990ae6c1bad32952e371fa72bee65a7af11c313 See https://bugs.kde.org/show_bug.cgi?id=388759#c14 for further details.
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Well, meanwhile the patched / fixed Qt (and its successors) was obviously included in Neon.