I can describe the problem only as an example.
After reboot system tray works as it should. I can left-click once on each icon to open network or sound control. The proper icon in tray is highlighted after such click.

For example I listen to music using deadbeef player and often change music volume using tray icon. Left click on tray's sound icon, move slider, click icon (or desktop) again to close widget.

After a few tens of such adjustments I notice that tray icon doesn't respond on the first click. Now I need two clicks to open sound settings.
On this stage sound settings looks normal but power manager looses battery charge indicator. To see the battery charge indicator, the battery icon must be clicked 3 times.
But still, tray is usable unless I click "triangle" icon which opens "status and notifications".
After that each click (sometimes double, sometimes single) on such tray icons as "battery", "network", "clipboard", "sound" opens "status and notifications" widget. The whole tray is highlighted with blue line.

But!
- clock widget, keyboard layout icon and third-party icons such as thunderbird, goldendict work properly. 
- right-click works properly. I can open "configure networks", "configure power saving" etc
- I can unlock widgets and open "system tray settings" which show absolutely empty "Entries" page. 

I couldn't find any workaround or the way to restart only the system tray.