The toggle only does one thing
Turning on the system proxy makes the client write one setting into your operating system: send traffic to the mixed port on localhost. Nothing more. So when it misbehaves, either something is wrong at the port, or something else is fighting over that same setting.
The mixed port defaults to 7897 on current builds and 7890 on configurations carried over from older ones. Confirm which number you are actually using before you start, because every step below refers to it.
Never run two proxy clients at once
They overwrite the same OS setting, the last writer wins, and the behaviour becomes impossible to reason about.
Match the symptom to the cause
- 1
The switch turns itself back off
Usually the port is taken, and the client refuses to point the OS at a listener that failed to start. Search the log page for a port related error, then change the mixed port in settings to something unused, 7898 instead of 7897 for example, and flip the switch again.
- 2
Enabled in the app, disabled in the OS
Something is reverting it. System tuning utilities, corporate management agents and rival proxy clients have all been caught doing this. Close them and retry. If the culprit is management software you cannot remove, stop using the system proxy and switch to TUN mode instead.
- 3
The app is closed and nothing loads
The proxy setting was left enabled and your system is still sending traffic to a port that no longer exists. Clear it by hand: on Windows turn off manual proxy setup in the proxy settings page, on macOS uncheck each protocol in the network proxy list. Connectivity returns immediately.
Confirm the change reached the OS
Do not trust the client interface alone. Open the operating system proxy settings and look at the address and port with your own eyes. It should read 127.0.0.1 followed by whichever port you are running. If the client claims it is on and the OS field is empty, something overwrote it and you are back at step two.
- The port in the OS proxy settings matches the port in the client
- Quitting the client flips that OS setting back to off on its own
- No browser level proxy setting or extension is overriding the system value
Common questions
It says it cannot set the system proxy because of permissions. Now what?
Try launching the client once as an administrator. If that changes nothing, group policy or a configuration profile has locked the proxy settings, which you can confirm because the fields in the OS settings page will be greyed out. No client can write there, so this one goes to whoever manages the device.
After closing the client the browser still says a proxy is in use.
The process may genuinely still be running in the background. Quit properly from the tray menu, then clear the OS proxy setting by hand. Force killing a process skips the cleanup step, and that is exactly where these leftovers come from.