For both Psi+ (Qt4) and HPLIP Status Service, double-clicking does nothing. At least Psi+ has well-defined double-click actions which worked fine in KDE 4. Right-clicking works fine. I am unsure if left-click has a defined action, but it also does nothing. Reproducible: Always
Double click is not part of the SNI spec. https://www.freedesktop.org/wiki/Specifications/StatusNotifierItem/StatusNotifierItem/ PSI is using that to talk to the system tray. If that is using double click, then that application has a bug.
Double-click is expected UI behaviour for systray icons. How are applications supposed to handle it? This worked fine in KDE 4...