SUMMARY Starting a bridge connection with a slave through the nm applet (or "Connections" in system settings) makes the connection stuck at "connecting". STEPS TO REPRODUCE 1. Create a bridge connection using the applet, and add an ethernet device as a slave. Using default settings will create a slave w/ DHCP. You may want to disable IPv6 and STP here, because, in some setups, they make the bridge connection hang for a while. 2. "Connect" to the bridge using the applet. OBSERVED RESULT The status of the bridge connection in the applet is stuck at "Connecting" indefinitely without any errors nor warnings. EXPECTED RESULT The bridge becomes successfully "Connected". SOFTWARE/OS VERSIONS Linux/KDE Plasma: Arch Linux (available in About System) KDE Plasma Version: 5.18.5 KDE Frameworks Version: 5.70 Qt Version: 5.14.2 ADDITIONAL INFORMATION The problem here is that the applet does NOT start the slave along with the bridge. Starting the slave manually (i.e. `nmcli connection start <name-of-the-slave>`) does fix the issue. The applet isn't malfunctioning, but is merely misaligned with the underlying service. Maybe the solution for this is to simply include slaves in the connection list, just like nmcli. It doesn't matter if clicking on them only shows a stub message (instead of configuration form), because what's important is that all connections are visible to users.
I believe this situation still stands today with Plasma 5.20.5. Although I've run only once in this specific problem (enslaved connection not being started), it is odd not to see (nor being able to manage) slaves in the applet and editor, especially since they appear when hovering the icon, and we are notified when they connect/disconnect. Thus I support Eon's proposal: why not include slaves in connection list and editor, as they are in nmcli?
I am affected by this in plasma 5.23.4 / frameworks 5.88.0 on Arch Linux. I needed to create a bridge for KVM virtual machine and couldn't do it with the KDE networkmanager applet. I ended up using nm-connection-editor to set it up. Now the plasma tray icon just shows a question mark and no connections listed in the applet. But when I hover the tray icon it shows "Wired Ethernet: connected to host" which is my slave connection on the bridge.
Still broken with 5.25/5.99. Workaround is to reboot, so annoying.