I experience this issue, too. I have a OnePlus 6 and Samsung Galaxy S8, both with Android 9. The Samsung is behaving as expected, while the OnePlus has the keyboard cover the lower half of the transmitted screen and has the toolbar at the bottom with the phone's navigation buttons disappearing. By switching the app back and forth, or swiping up, I get the buttons back. Then I am able to hide the keyboard to see the lower half of the transmitted screen. Both phones are set up to use on screen navigation buttons, and use swiftkey as keyboard. The issue doesn't seem to be linked to type of navigation (gestures or soft buttons) or the selected keyboard, and is also independent of the device it is connected to.. This issue is present at least for the past year, and even now with the reworked app toolbar in version 15.0.13, it seems to persist. I am more than willing to troubleshoot or assist in finally fixing this, as clearly this issue is not just due to OnePlus being "exoctic".
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