Created attachment 143752 [details] recording demonstrating what happens and how it fits whitin the the "thickness" setting from latte SUMMARY I've attached a gif demonstrating exactly what happens. This does not happen on horizontal panels. The tray subitem seems to ignore completely the 'thickness' setting within latte when drawing the popup. ADDITIONAL INFORMATION I'm on wayland, but that happens on X11 too. Other than that, thank you for your work :)
This is systray fault... Since Plasma 5.21 it is applets responsibility to request the entire thickness of then panel. If you try plasma panel and plasma systray you will get the exact same results. If Plasma systray wants the entire thickness should set: Plasmoid.constraintHints: PlasmaCore.Types.CanFillArea for this and many other tweaks such as icon spacing, icon backgrounds etc... I have implemented and you can use: https://github.com/psifidotos/plasma-systray-latte-tweaks it provides you an option to set: Fill all panel thickness with no margins.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1238
Discussion in https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1238 indicates that PlasmaCore.Types.CanFillArea is inappropriate for the System Tray because we *do* want it to follow panel margins.