When I open the drawer for hidden system tray items I'm not seeing all of them, unless I pin and resize with Alt+RightClick the pop-up (see the attached screenshots). The area however is not scrollable with a mouse. On this machine I have a touchscreen and I am able to scroll this area *only* with it.

This makes me almost sure that the issue is due to this bug in Qt: https://bugreports.qt.io/browse/QTBUG-41591. We've suffered from that before and fixed it by no longer hiding scrollbars  automatically after some timeout even when touchscreen is available: https://quickgit.kde.org/?p=plasma-framework.git&a=commit&h=01d7343642543b75f90ee43d4ed347f9fdd7b0e9

However, I still suffer this, because (not sure, correct me if I'm wrong here) there are no initial scrollbars of the system tray pop-up to be hidden in the first place.

Reproducible: Always

Steps to Reproduce:
1.  Make sure you have working touchscreen.
2. Open a lot of applications that have icons in the system area.
3. Click on the up arrow in order to show hidden tray items

Actual Results:  
Observe that not all of the items are visible, there is no scrollbar and there is no way to scroll with the mouse. However it is possible to scroll via the touchscreen or pin and resize the pop-up.

Expected Results:  
Pop-up should be scrollable with a mouse regarding of the availability of a touchscreen. It would be also nice to have a visible scrollbar. I didn't think of trying to scroll with the touchscreen before I found BUG: 347254, while trying to report this one.

At first I thought this is due to the fact that some of the applications in my system tray are using the old deprecated system tray mechanisms. However, as you can see on the screenshots, I am able to reproduce this with pure KF5 applications.