Bug 417565

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.