If you open links in some Android apps, and you have Chrome enabled as your default web browser, the page might be opened in a ‘Chrome Custom Tab.’ Instead of starting up the whole app, only a minimal browser UI is shown, with a large close button that takes you right back to the previous app. This feature requires the app open links a certain way, and as such, there have always been some apps that don’t support Custom Tabs.
Chrome Beta 65 removes flag that opens all app links as Chrome Custom Tabs was written by the awesome team at Android Police.