When the System Tray widget is added to the desktop, it enforces two rows of icons. It can't be resized in a way that only one row is shown. Prior to 4.10 Beta, the widget had problems with being resizeable too much, now it can be resized too little.
Added Dmitry, who did the bulk of the work to port the systemtray to QML.
Hello! This bug report was filed for KDE Plasma 4, which reached end-of-support status in August 2015. KDE Plasma 5's desktop shell has been almost completely rewritten for better performance and usability, so it is likely that this bug is already resolved in Plasma 5. Accordingly, we hope you understand why we must close this bug report. If the issue described here is still present in KDE Plasma 5.12 or later, please feel free to open a new ticket in the "plasmashell" product after reading https://community.kde.org/Get_Involved/Bug_Reporting If you would like to get involved in KDE's bug triaging effort so that future mass bug closes like this are less likely, please read https://community.kde.org/Get_Involved#Bug_Triaging Thanks for your understanding! Nate Graham