Fix scrollToObject: bug when scrolling to bottom with content inset

Summary:
This has been around forever, surprised we've never run into it. Instagram doesn't really use the `bottom` position except for one place that is already at the bottom of a view. Looks like this bug has just been hanging around quietly for ages.

`UIScrollView` takes into account the content inset when changing the content offset. Doing math w/ the inset is both useless and incorrect.

Issue fixed: #932

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
Closes #940

Differential Revision: D5881117

Pulled By: rnystrom

fbshipit-source-id: 5c9af4b6f8b59be977491ba734e9b0fa3c538cb9

CHANGELOG.md

@@ -13,15 +13,12 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 - Weakly reference the `UICollectionView` in coalescence so that it can be released if the rest of system is destroyed. [Ryan Nystrom](https://github.com/rnystrom) [(#tbd)](https://github.com/Instagram/IGListKit/pull/tbd)
 
+- Fix bug with `-[IGListAdapter scrollToObject:supplementaryKinds:scrollDirection:scrollPosition:animated:]` where the content inset of the collection view was incorrectly being applied to the final offset. [Ryan Nystrom](https://github.com/rnystrom) [(#tbd)](https://github.com/Instagram/IGListKit/pull/tbd)
+
 - Avoid crash when invalidating the layout while inside `-[UICollectionView performBatchUpdates:completion:]. [Ryan Nystrom](https://github.com/rnystrom) [(#tbd)](https://github.com/Instagram/IGListKit/pull/tbd)
 
 - Duplicate view models in `IGListBindingSectionController` gets filtered out. [Weyert de Boer](https://github.com/weyert) [(#916)](https://github.com/Instagram/IGListKit/pull/916)
 
-### Enhancements
-
-- Added `-[IGListSectionController didHighlightItemAtIndex:]` and `-[IGListSectionController didUnhighlightItemAtIndex:]` APIs to support `UICollectionView` cell highlighting. [Kevin Delannoy](https://github.com/delannoyk) [(#933)](https://github.com/Instagram/IGListKit/pull/933)
->>>>>>> source: 872ef395b114 arcpatch-D5872117 - facebook-github-bot: [IGList...
-
 3.1.1
 -----
 

Source/IGListAdapter.m

@@ -275,7 +275,7 @@ - (void)scrollToObject:(id)object
         case UICollectionViewScrollDirectionVertical: {
             switch (scrollPosition) {
                 case UICollectionViewScrollPositionBottom:
-                    contentOffset.y = offsetMax - collectionViewHeight - contentInset.top;
+                    contentOffset.y = offsetMax - collectionViewHeight;
                     break;
                 case UICollectionViewScrollPositionCenteredVertically: {
                     const CGFloat insets = (contentInset.top - contentInset.bottom) / 2.0;

Tests/IGListAdapterTests.m

@@ -714,6 +714,34 @@ - (void)test_whenScrollVerticallyToItemWithPositionning {
     IGAssertEqualPoint([self.collectionView contentOffset], 0, self.collectionView.contentSize.height - self.collectionView.frame.size.height);
 }
 
+- (void)test_whenScrollVerticallyToBottom_withContentInsets_thatBottomFlushWithCollectionViewBounds {
+    self.dataSource.objects = @[@100];
+    [self.adapter reloadDataWithCompletion:nil];
+
+    // no insets
+    self.collectionView.contentInset = UIEdgeInsetsMake(0, 0, 0, 0);
+    [self.collectionView layoutIfNeeded];
+    [self.adapter scrollToObject:@100 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionBottom animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 900);
+
+    // top 100
+    self.collectionView.contentInset = UIEdgeInsetsMake(100, 0, 0, 0);
+    [self.adapter scrollToObject:@100 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionBottom animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 900);
+
+    // bottom 100
+    self.collectionView.contentInset = UIEdgeInsetsMake(0, 0, 100, 0);
+    [self.collectionView layoutIfNeeded];
+    [self.adapter scrollToObject:@100 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionBottom animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 900);
+
+    // top 50, bottom 100
+    self.collectionView.contentInset = UIEdgeInsetsMake(50, 0, 100, 0);
+    [self.collectionView layoutIfNeeded];
+    [self.adapter scrollToObject:@100 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionBottom animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 900);
+}
+
 - (void)test_whenScrollHorizontallyToItem {
     // # of items for each object == [item integerValue], so @2 has 2 items (cells)
     IGListTestAdapterHorizontalDataSource *dataSource = [[IGListTestAdapterHorizontalDataSource alloc] init];