Just updated my Slackware 14.2, KDE/Plasma 5.17.11 (with NoMachine supplied startSession.sh) to the latest 6.8.1.
Following this, when opening Firefox, Thunderbird, I no longer have a title bar, with the window controls. Also cannot select an edge of the window to drag and resize. Pull down menus display, but moving the cursor to select an option closes the menu.
Fired up the ESXi console, started a native KDE session, and everything works correctly there.
I have the exact same issue on Kubuntu and KDE Neon. Also here – it worked fine with 6.8.1. Just wanted to let you know that something did indeed change and to me it does act up like a bug (unexpected behavior).
Also – tried using the suggestion above but replacing kwin_x11 from root leads to incorrect settings loading, defaulting to higher dpi so I suppose that’s not really right solution here. Downgraded to 6.7.6_14 amd64 for the time being.
In addition to what I wrote above, it obviously does not work fine with 6.8.1 — it did work fine with the earlier 6 series releases.