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Found the issue, ugh

Noticed that when I do the following:
transaction.addToBackStack(null)

When adding a Fragment with ViewPager2, the above issue occurs, although, idk if it's becaugh of gcFragments() anymore.

As soon as I un-comment that line, the issue disappears.

https://github.com/PenguinDan/ViewPagerCrash

Is the Sample application, to reproduce, move back really fast between page 2 -> 1 and vice versa (they are bottom buttons) a couple times.
Wait a couple seconds and page 1 (the black page) should turn white meaning the Fragment has been removed.

As soon as you comment line 40 on MainActivity, and attempt again, the error does not persist

Woops, sorry, If you guys had the previous ViewPagerCrash test app, I forgot to push (was exhasuted after debugging) I pushed one more time for the latest reproducible code

We'll have a look, thanks for reporting and repro steps / sample app. Stay tuned for updates on this issue.

Got to debug this now. Your sample indeed showcases an issue, so thanks for reporting and for the sample!

`transaction.addToBackStack(null)` adds the Fragment replacement transaction to the back stack, so you can go back to the previous state with the back button. Is that what you were looking to achieve? You said it works without it, which might help in the meantime.

Would you mind describing the use-case, so I better understand what you're trying to achieve? E.g. in the example, by switching between buttons 1 and 2 at some point you achieve multiple stack entries going back to the ViewPager2 Fragment.

Mostly for my notes, but the trace below explains what happens. Still need to work out why.

We have 5 instances of FragmentStateAdapter below.
Notice how in lines 36-39 4 different instances of FragmentStateAdapter perform a scheduled gc (after grace period expires).
Those don't have the Fragments bound to their ViewHolders as those were removed: lines  8, 16, 24, 32.

At the same time, com.test.myapplication.VPAdapter.createFragment(ViewPagerFragment.kt:161) only gets called once, so presumably all 5 have a reference to that same Fragment.

Needs more investigation as looks fishy.

     1  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be modified at FragmentStateAdapter.<init>(FragmentStateAdapter.java:85). Current value = 'null'. New value = 'instance of androidx.collection.LongSparseArray(id=11289)'
     2  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
     3  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onBindViewHolder(FragmentStateAdapter.java:174)
     4  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
     5  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:235)
     6  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:240)
     7  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
     8  {FragmentStateAdapter@11,288}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onViewRecycled(FragmentStateAdapter.java:391)
     9  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be modified at FragmentStateAdapter.<init>(FragmentStateAdapter.java:85). Current value = 'null'. New value = 'instance of androidx.collection.LongSparseArray(id=11307)'
    10  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    11  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onBindViewHolder(FragmentStateAdapter.java:174)
    12  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    13  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:235)
    14  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:240)
    15  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    16  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onViewRecycled(FragmentStateAdapter.java:391)
    17  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be modified at FragmentStateAdapter.<init>(FragmentStateAdapter.java:85). Current value = 'null'. New value = 'instance of androidx.collection.LongSparseArray(id=11314)'
    18  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    19  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onBindViewHolder(FragmentStateAdapter.java:174)
    20  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    21  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:235)
    22  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:240)
    23  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    24  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onViewRecycled(FragmentStateAdapter.java:391)
    25  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be modified at FragmentStateAdapter.<init>(FragmentStateAdapter.java:85). Current value = 'null'. New value = 'instance of androidx.collection.LongSparseArray(id=11316)'
    26  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    27  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onBindViewHolder(FragmentStateAdapter.java:174)
    28  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    29  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:235)
    30  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:240)
    31  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    32  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onViewRecycled(FragmentStateAdapter.java:391)
    33  {FragmentStateAdapter@11,317}.mItemIdToViewHolder will be modified at FragmentStateAdapter.<init>(FragmentStateAdapter.java:85). Current value = 'null'. New value = 'instance of androidx.collection.LongSparseArray(id=11318)'
    34  {FragmentStateAdapter@11,317}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.itemForViewHolder(FragmentStateAdapter.java:234)
    35  {FragmentStateAdapter@11,317}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.onBindViewHolder(FragmentStateAdapter.java:174)
    36  {FragmentStateAdapter@11,306}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.gcFragments(FragmentStateAdapter.java:221)
    37  {FragmentStateAdapter@11,313}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.gcFragments(FragmentStateAdapter.java:221)
    38  {FragmentStateAdapter@11,315}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.gcFragments(FragmentStateAdapter.java:221)
    39  {FragmentStateAdapter@11,317}.mItemIdToViewHolder will be accessed at FragmentStateAdapter.gcFragments(FragmentStateAdapter.java:221)

Pausing the investigation for now until we better understand the use-case -- questions in comment#6

Yes, completely correct that I am attempting to use the back pressed implementation but also a bottom navigation implementation to switch to and from Fragments.

Currently the app that I am trying to make contains multiple Fragments with and without ViewPager2's accessible through a bottom navigation set of buttons. Through simple usage of moving to and from my ViewPager2 Fragments, I would see random instances of child Fragments disappearing from them.

I'd rather like to keep the "addToBackStack(..)" to rewind through fragment transactions. Will definitely look further into it also from the above mentioned logs.