Hello there and thank you for your work. After upgrading to f28, I've noticed a minor change in mate-panel's Notification Area (aka systray) behaviour when operating apps' icons with mouse. In fact, there is only one app that was broken: Telegram. Before upgrade, left-click on the icon popped up the app, another left-click closed the app (normal traditional behaviour). Now left-click don't work at all: happens nothing. Right-click shows Context Menu of the app (as normal). Thanks Fedora, the rest of systray seem to work well in f28. Other apps I tried (Clementine and Exaile) work as usual (before upgrade), sound and network control works also as usual. The problem may come from MATE itself, because there are much bigger troubles with the same tray in recent Ubuntu-MATE. Ubuntu-MATE, after recent upgrade to 18.04, broke the MATE systray in much worse way: broken are both Telegram, and Clementine, and sound control, and network control (I've reported https://bugs.launchpad.net/ubuntu-mate/+bug/1769856). Fedora is much better here, and I hope this Telegram systray flaw can be easily fixed so that I can upgrade more PCs to f28.
With f28 i enabled this gsettings key [rave@satellite ~]$ gsettings get org.mate.panel enable-sni-support false I guess this is the main different to ubuntu. But you can try to disable this key in fedora In general this is an upstream issue, i can't fix it here.
THANK YOU!!! Setting this to false an rebooting worked for me. I am so happy!
I am lucky too that i could help you. Btw. restarting panel is sufficient if you change this gsettings key. 'killall mate-panel` Panel is controlled by session-manager which do the restart if an app crashes.