• Restore the tab pager adapter state after populating the list of tabs and avoid navigating to input screen on fragment restoration #8180 👈 (View in Graphite)
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@LukasPaczos I have further investigated this issue and found out why this happens. We have our custom FragmentStateAdapter, which uses hide()/show() to keep fragments alive instead of removing and recreating them (the default library behavior). Fragments are added without a container ID (tag-only add()), so Fragment.mContainer is null.

The issue involves three pieces of the Fragment framework interacting:

1. hideFragment sets mHidden = true
   When onViewRecycled calls hideFragment, it runs: mFragmentManager.beginTransaction().hide(fragment).commitAllowingStateLoss();

FragmentManager.hideFragment() sets fragment.mHidden = true. During normal swiping, the view stays VISIBLE, the fragment is just hidden at the FragmentManager level.

2. Tab switcher triggers a lifecycle transition that sets the view to GONE
   When the tab switcher activity opens the BrowserTabFragment is destroeyd (due to “Don’t keep activities being enabled). When we go back and select one of the tabs at the edge, which has been hiden before due to holder being recycled (and FragmentManager.hideFragment() called), we go through the onCreateView stage (as the fragment view is being createaed), inside FragmentStateManager.createView() which runs:

if (mFragment.mHidden) {
    mFragment.mView.setVisibility(View.GONE);
}

Since mHidden is true, the view is set to GONE. This doesn't happen during normal swiping because the fragment is just hidden and the view is not being destroyed.

3. show() can't undo the GONE visibility
   When returning from the tab switcher, showFragment calls show(fragment).commitNow(). This sets mHidden = false, but the SpecialEffectsController, which is responsible for calling view.setVisibility(View.VISIBLE) is skipped because mFragment.mContainer is null (fragments are added with a tag only, no container ID):

// FragmentStateManager.java
if (mFragment.mHiddenChanged) {
    if (mFragment.mView != null && mFragment.mContainer != null) {  // skipped — mContainer is null
        SpecialEffectsController controller = ...
        controller.enqueueShow(this);  // never reached — this is what sets VISIBLE
    }
}

Result: mHidden = false but the view is still GONE.

The solution is to explicitly set the view to VISIBLE in showFragment, since FragmentManager cannot do it when mContainer is null:

private void showFragment(long itemId) {
    Fragment fragment = mFragments.get(itemId);
    if (fragment == null) return;

    if (fragment.isAdded() && fragment.isHidden()) {
        mFragmentManager.beginTransaction().show(fragment).commitNow();
    }
    if (fragment.getView() != null && fragment.getView().getVisibility() != View.VISIBLE) {
        fragment.getView().setVisibility(View.VISIBLE);
    }
}

This was not an issue before because gcFragments would remove the framents (as the state was not being handled correctly (the tabs in TabPagerAdapter were not initialised so containsItem(itemId) was false, therefore the fragment was removed).

I will push a commit with the fix.
CC: @0nko

@catalinradoiu Really great investigation! 👏 Must not have been easy to pinpoint the culprit behind the behavior..

I've looked over the changes and don't see any problems, aside from the BugBot comment, which I think is legitimate.

@catalinradoiu The changes look good to me. I've tested it and didn't find any issues.

@0nko @LukasPaczos I have pushed 2 more commits to make the feature flag enabled by default and move the selectedTabObserver?.cancel() call that was flaged in the comment above. I will merge the PR once the CI passes.