Fix overlapping items after off-screen updates

[bryankeller]

Details

This PR fixes an issue that causes off-screen batch updates to sometimes cause visual defects in the form of temporary overlapping items. It's probably a UICollectionView bug, but thankfully, there's a way for MagazineLayout to work around it. Here's the technical description straight from the in-code documentation:

This prevents an issue that causes overlapping / misplaced elements after an
off-screen batch update occurs. The root cause of this issue is that UICollectionView
expects layoutAttributesForElementsInRect: to return post-batch-update layout attributes
immediately after an update is sent to the collection view via the insert/delete/reload/move
functions. Unfortunately, this is impossible - when batch updates occur, invalidateLayout:
is invoked immediately with a context that has invalidateDataSourceCounts set to true.
At this time, MagazineLayout has no way of knowing the details of this data source count
change (where the insert/delete/move took place). MagazineLayout only gets this additional
information once prepareForCollectionViewUpdates: is invoked. At that time, we're able to
update our layout's source of truth, the ModelState, which allows us to resolve the
post-batch-update layout and return post-batch-update layout attributes from this function.
Between the time that invalidateLayout: is invoked with invalidateDataSourceCounts set to
true, and when prepareForCollectionViewUpdates: is invoked with details of the updates,
layoutAttributesForElementsInRect: is invoked with the expectation that we already have a
fully resolved layout. If we return incorrect layout attributes at that time, then we'll have
overlapping elements / visual defects. To prevent this, we can return nil in this
situation, which works around the bug.
UICollectionViewCompositionalLayout, in classic UIKit fashion, avoids this bug / feature by
implementing the private function
_prepareForCollectionViewUpdates:withDataSourceTranslator:, which provides the layout with
details about the updates to the collection view before layoutAttributesForElementsInRect:
is invoked, enabling them to resolve their layout in time.

Before	After

Before	After

Related Issue

N/A

Motivation and Context

This works around an old issue that's affected MagazineLayout since day one. Also, I love a good UICollectionView bug.

How Has This Been Tested

• Tested in sample project
• Tested in Airbnb app

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[brynbodayle]

Great stuff, thanks for fixing this!