I've got a small filtering proxy in our home LAN, used as a web filter for my kids. One most annoying mis-feature is that every time a Linux computer with a "modern" UI such as GNOME connects to the network, a popup window asks the user to enter "tickets" for my Fritz! box, although NM correctly sets up the proxy and net surfing is possible just fine. Currently the only workaround for this is to deactivate NM's "connectivity check" altogether. IMO that's not the right solution - if NM is able to detect and configure proxies for the system, it should also be able to use these proxy settings for its connectivity check.

This touches on the fact that proxy configuration support is generally in a miserable state (not only) on Linux.

1. NM needs pacrunner for setting up connection-specific proxies, either autodetected or manually configured. package pacrunner for SUSE. There are some security concerns involved, e.g. pacrunner runs as root by default. I have a prototype pacrunner package that fixes this.
2. libproxy needs integrated pacrunner support. It has had this for years, if not decades, under Fedora, but it hasn't been integrated upstream. I've created a PR but it may need more discussion / work.
3. NM should use the detected proxy for connectivity check, at least optionally (see above)
4. Usage of libproxy should be advocated more.

For hackweek 19, I'd like to look into 1., 2., and possibly, 3.