SUMMARY Clicking on a panel item (like the menu, calendar, wifi indicator, etc...) and closing it by clicking its icon again or by pressing escape breaks temporarily gtk apps scrolling. STEPS TO REPRODUCE 1. Open a gtk app like evince or gedit with some file and resize it at a size where scrolling is available. 2. Focus the gtk app and verify that scrolling works 3. Click on calendar on plasmashell's panel and then close by either clicking on the calendar icon again or by hitting escape. 4. Then click on the open gtk app to focus it again and see that scrolling no longer works OBSERVED RESULT Scrolling breaks if you close panel's menus by clicking on their icons or by hitting escape. If instead you click on the gtk app after you have opened a panel's menu (without closing it first) then scrolling works properly. If you switch windows with alt-tab or with the panel's task manager then scrolling works again. EXPECTED RESULT Scrolling should work when selecting the gtk app again no matter how you closed a panel's menu. SOFTWARE/OS VERSIONS Linux/KDE Plasma: arch linux KDE Plasma Version: 5.18.0 KDE Frameworks Version: 5.67.0 Qt Version: 5.14.1 ADDITIONAL INFORMATION I had noticed the same behavior with plasma 5.17 too but I waited for the new release to verify that it's not something it got fixed.
Sadly this is a GTK bug that the developers seem unable or unwilling to fix. :( See https://bugzilla.gnome.org/show_bug.cgi?id=750870 and https://gitlab.gnome.org/GNOME/gtk/issues/558 One workaround is to use Wayland. Apparently the bug only happens on X11.