The "Capture" button on the Stadia controller (pictured just above, it's the one with the square brackets near the center) now actually does what it's supposed to do when you're using it on an Android device. A recent update has rolled out the ability for it to actually capture screenshots and clips on Android phones.
The news was originally spotted yesterday on Reddit and confirmed today in a blog post from Google. Previously, the button didn't do anything on mobile; it just prompted an error pop-up saying to try on a TV or in your browser. Until now, to grab a screen when playing on mobile, you had to use your phone's screenshot shortcut — hardly convenient. The update brings Stadia one step closer to feature parity across screens.
Google has also confirmed that the OnePlus 8 and OnePlus 8 Pro are the first OnePlus phones compatible with Stadia, joining a catalog of devices from Google, Samsung, Asus, and Razer.