Thanks to https://phabricator.kde.org/T6818, the gtk3-nocsd package will be present in default Kubuntu installations, substantially improving the user experience with CSD-decorated apps ...most of them. There are a few that aren't affected by the changes that package makes, and unfortunately one of my favorites--Lollypop--is among them. Happily, Lollypop includes an internal option to disable the headerbar, which has the effect of doing the same thing. This makes Lollypop look and behave perfectly when run under KWin.
We should adjust the lollypop package in Kubuntu to add the following command:
gsettings set org.gnome.Lollypop disable-csd true
This packaging change should only be made for Kubuntu, not regular Ubuntu (where the CSD style works fine because they don't use KWin as the window manager).