The progress bar widget has wrong window flags, making it have its own task bar entry and misbehave on tiling WMs. This is a regression from 5.0, probably in KF5 code.
Git commit 115cacb46905e35f81c495894ee85272a593aed2 by Sven Brauch. Committed on 30/12/2016 at 22:18. Pushed by brauch into branch '5.1'. fix progress widget flags M +1 -1 shell/progresswidget/overlaywidget.cpp https://commits.kde.org/kdevplatform/115cacb46905e35f81c495894ee85272a593aed2