There are several background window entries that are highlighted in the Task Manager. The only foreground window entry is suppose to be highlighted. Please see the screenshot for the problem. Background forum thread - https://forum.kde.org/viewtopic.php?f=285&t=131982 Reproducible: Always
Created attachment 98289 [details] task manager screenshot
Plasma 5.7 ships a fully-rewritten Task Manager backend which should resolve these issues. Please re-test with the Plasma 5.7 final release and feel free to reopen if problems do persist after all.