IssueTracker

I can't reproduce this issue myself but I notice this issue in production with Xiaomi MI 9 running `Android 10`. Here are the `ViewPager2` and `RecyclerView` versions I use:

    androidx.recyclerview:recyclerview:1.1.0
    androidx.viewpager2:viewpager2:1.0.0

And here's the stacktrace:

```
Fatal Exception: java.lang.IllegalStateException: Design assumption violated.
       at androidx.viewpager2.adapter.FragmentStateAdapter.placeFragmentInViewHolder(FragmentStateAdapter.java:287)
       at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(FragmentStateAdapter.java:276)
       at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(FragmentStateAdapter.java:67)
       at androidx.recyclerview.widget.RecyclerView.dispatchChildAttached(RecyclerView.java:7556)
       at androidx.recyclerview.widget.RecyclerView$5.addView(RecyclerView.java:860)
       at androidx.recyclerview.widget.ChildHelper.addView(ChildHelper.java:107)
       at androidx.recyclerview.widget.RecyclerView$LayoutManager.addViewInt(RecyclerView.java:8601)
       at androidx.recyclerview.widget.RecyclerView$LayoutManager.addView(RecyclerView.java:8559)
       at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1643)
       at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1587)
       at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:675)
       at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:4134)
       at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3851)
       at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4404)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at androidx.viewpager2.widget.ViewPager2.onLayout(ViewPager2.java:527)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
       at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
       at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at com.android.internal.policy.DecorView.onLayout(DecorView.java:803)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:3144)
       at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2654)
       at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1782)
       at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7785)
       at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1031)
       at android.view.Choreographer.doCallbacks(Choreographer.java:854)
       at android.view.Choreographer.doFrame(Choreographer.java:789)
       at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1016)
       at android.os.Handler.handleCallback(Handler.java:883)
       at android.os.Handler.dispatchMessage(Handler.java:100)
       at android.os.Looper.loop(Looper.java:224)
       at android.app.ActivityThread.main(ActivityThread.java:7520)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
```

I'm not sure where this is triggered from my code since the stacktrace is not very clear, so I can't create a sample project with the bug for you to have a look at. But if it's helpful, here's my implementation of the adapter:

```
class MyAdapter(private val data: List<Data>, private val fragmentActivity: FragmentActivity)
    : FragmentStateAdapter(fragmentActivity) {

    private val itemIds = mutableSetOf<Long>()
    private var registeredFragments = SparseArray<Fragment>()

    override fun getItemCount(): Int = data.size
    override fun createFragment(position: Int): Fragment {
        val fragment = FileRenderHelper.findAppropriateFragment(data[position], position)
        registeredFragments.put(position, fragment)
        return fragment;
    }

    override fun getItemId(position: Int): Long {
        // Returning a new itemId so we don't store the old pages when replacing files.
        // Source: https://stackoverflow.com/a/11710690/2997806
        return when (val data = data[position]) {
            is CustomClass -> data.id
            else -> data.hashCode().toLong()
        }.also { itemIds.add(it) }
    }

    override fun containsItem(itemId: Long): Boolean = itemIds.contains(itemId)

    fun getFragmentAt(position: Int): Fragment? {
        return registeredFragments[position]
    }
}
```


 Also, as per a similar issue, this has been solved in viewpager2:1.0.0-beta03: https://issuetracker.google.com/issues/134246546 with the commit [here][1] but not quite in my case. And here's another post with the same issue with not many helpful answers: https://stackoverflow.com/questions/57352437/fragmentstateadapter-throws-java-lang-illegalstateexception-design-assumption-v. It'd be great if anyone knows why this occurs or how it can be solved.

  [1]: https://android-review.googlesource.com/c/platform/frameworks/support/+/1092796/2/viewpager2/src/main/java/androidx/viewpager2/adapter/FragmentStateAdapter.java#b403