the item height must be less than the height of the UICollectionView minus the section insets top...
ThebehavioroftheUICollectionViewFlowLayoutisnotdefinedbecause:2020-12-1310:32:16.544810+0800MyBox[1073:195016]theitemheightmustbelessthantheheightoftheUICollectionViewminusthesectioninsetstopandbottom