Created attachment 140095 [details] Opacity is not applied to Tray widgets and Widgets Adder If you set panel opacity to Opaque then it will not be applied to Tray widgets and Widgets Adder. See attached image for more info.
Seems like a bug; panel widgets other than the System Tray become opaque, but the System Tray does not. As for the Add Widgets sidebar, that's intentional because it's not connected to the panel itself. The Panel opacity setting is only supposed to concern the panel and any widgets on it. It's not a general "turn off all transparency in Plasma" button.
The problem is not specific to only Tray system widgets, all "Show Alternatives" dialogs, desktop notifications, brightness/audio osd suffer from this problem. See attached image for more info.
Created attachment 140120 [details] Opacity problems
Also the panel edit mode is affected.
Created attachment 140121 [details] Panel edit mode opacity
Created attachment 140122 [details] Panel edit mode opacity 2
Like ahe Add Widgets sidebar, desktop notifications and status OSDs are not connected to a panel so they are not expected to respect the per-panel transparency setting. Again, this is a *per-panel* setting, not a general transparency toggle for all of plasmashell. However the Alternatives popup and the Edit Mode UI elements are connected to a panel, so it would make sense for them to respect the opacity setting. Can you file a new bug report in plasmashell | panel for those? If you could also mark it in the "See Also" field of this bug report, that would be lovely. :)
*** This bug has been marked as a duplicate of bug 439025 ***