UICollectionView list layout with missing cells

An example project with a UICollectionViewLayout subclass for a UITableView-style layout, illustrating some problems.

If you run the project and scroll down in the list, some cells are missing. This is because:

• The estimated row height for the layout is set larger than cells will actually end up.
• The layout queries for items in a given rect, via layoutAttributesForElementsInRect.
• For each of those items, a cell gets built and sized, and its preferred size (which is smaller than the estimated height) gets applied to it and therefore the y offsets of all cells below it via shouldInvalidateLayoutForPreferredLayoutAttributes and invalidationContextForPreferredLayoutAttributes (followed by a call to prepareLayout).
• This pulls more cells into view within the rect that was originally asked about, but layoutAttributesForElementsInRect doesn't always get called again so the layout can answer based on its new cell positions.