When Android back button doesn't close your modals
A quick backstory from real life:
I opened my expen.app on a Xiaomi 12 (Android 15) and noticed the full-screen sheets don't close with the back button or edge swipe. On the Pixel 9 emulator (Android 16) though, it worked fine. What's going on?
After a bit of research, I came across OnBackInvokedCallback. Android 13 introduced it as a new way to dispatch back events, and it also powers "predictive back" (the edge-swipe gesture). For backward compatibility, Google made it opt-in. Adding this to AndroidManifest.xml fixed it for me:
Sheets, dialogs, modals on Xiaomi now close with back press or edge swipe.
Watch out: this flag breaks WillPopScope. Migrate to PopScope first (Flutter migration guide).
The part I still don't fully get: I have another Flutter app of mine on the same Xiaomi. Without the flag, its back button closes modals just fine. The logcat for that app shows the exact same warning:
W/WindowOnBackDispatcher: OnBackInvokedCallback is not enabled for the application.W/WindowOnBackDispatcher: Set 'android:enableOnBackInvokedCallback="true"' in the application manifest.
Same warning, same Android version, same Flutter version, MIUI delivering the back the same way. But one app's modals close, the other's don't. I tried bumping androidx.activity to match the working app's version, no change. Tried removing a custom Overlay wrapper, no change. Something in the expen.app's widget tree or plugins is consuming back differently and I couldn't track it down.
So if you hit this: add the flag, ship it. The fix works. The deeper cause goes beyond "Xiaomi gates the gesture", it apparently depends on your app's internals in ways I don't fully understand yet.
