Android is getting an API to seamlessly transition between screen sizes? Great! because right now it sucks at that.
Search for something in Keep. Rotate. The results are now blank.
Rotating in Gmail's compose screen will jump the "to" field.
Hangouts will close the keyboard.
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Android's support for transitioning screen states is *really* janky and works totally differently on an app-by-app basis. Some will even delete your data.
If we're going to be doing screen transitions more often with foldable phones, individual apps will need a lot more polish.
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It uses the generic support for saving / restoring activity state. If an app fails to handle this properly, it likely has the same failure for being pushed out of memory in the background and configuration changes other than screen size. Try opening a bunch of other apps to test.
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It's likely apps failing to handle rotation will lose exactly the same state if you open a dozen other apps to force them out of memory. Changing the locale or plugging in an external keyboard (at least with the virtual keyboard disabled) should also work as ways to trigger it.
App devs can set their activity as handling a list of configuration changes like rotation itself, which stops the activity from respawning. It makes rotation seamless but the app is still expected to handle saving/restoring activity state properly for cases like being killed.