Custom Tabs is a feature in Android that allows app developers to keep you in their app when you open a link in the browser. Custom tabs are different from the WebView UI feature in that they call to the default browser to render the page. While some users might see this as convenient, many do not, for these two reasons:
The visit is not stored in browser history.
Any subsequent pages visited or interactions are observed by the app, which is a privacy concern.
Some apps provide a switch in settings to turn this feature off, but many don't. In addition to several people online (one, two, three), I've spoken with several people I know personally who each independently told me that they would like to be able to turn this feature off.
This idea was posted on the Bugzilla 3 months ago, with no activity since. It would be easy for someone familiar with the code to add this feature, and I'd be willing to put up a bounty with others if there were someone willing to add this.