New answers tagged web-browser
Copy and paste chrome://settings/handlers into the address bar in Chrome and check for an active protocol handler for Gmail. If you don't find something there, you could try installing RCDefaultApp which should list any system wide protocol handlers.
It's something built in the system . Sometimes it's able to detect a captive portal and it displays it in a separate window. As @George Garside pointed out, it's a separate program located at /System/Library/CoreServices/Captive Network Assistant.app. Also, if you have a iDevice you'll see a similar feature.
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