In 2015, Google introduced Custom Tabs to Chrome on Android to provide a smooth transition from app to web content without resorting to a WebView. Compared to WebViews, the feature loads faster and allows apps to customize their web experience.
However, Custom Tabs do not have a default sharing experience on the app bar that users can tap to post content to their social media accounts, for example. This is compounded by the lack of a sharing option in most apps that use Custom Tabs.
That said, users can still find a share option by tapping on the overflow menu in the browser. In that case, however, users may have to leave the app and open the link in the browser, which reduces engagement for the app.
Google is trying to address this problem with a new experimental feature in Chrome 88. The browser is adding a default share action in certain conditions. It will show up in the top bar when an app has not set its own Action Button. In case an app already has an Action Button, a default share option will appear in the overflow menu.
The default share experience will be added automatically as long as the app doesn't provide an Action Button. That means users don't have to do anything to enable the new share action button. It remains to be seen when and if other browsers will adopt a similar feature.