Scroll to focused media below header

Summary:
## Context
We want the post-tap headers to be sticky so that context is maintained as the user scrolls down the media chain.

## In this diff
When clicking on a media tile in a media module (rather than the See All CTA), the sticky header was covering part of the media post's header. We want to shift the scroll-to view down so that the sticky header is sitting on top of the media post's header.

- Create new version of IGListAdapter API's `scrollToObject` function that contains an additional parameter `additionalYOffset`
- Call new `scrollToObject` function with offset of 48.0f (height of header)

## In this stack
Add sticky headers for post-taps, excluding SERP

Reviewed By: maxolls

Differential Revision: D29961626

fbshipit-source-id: c8644cc868e8d7ec92cad5f6c7742d93d001f67d

CHANGELOG.md

@@ -40,6 +40,25 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 - Introducing `allowsBackgroundDiffing` on `IGListAdapterUpdater`! This property lets the updater perform the diffing on a background thread. Originally introduced by Ryan Nystrom a while back. [Maxime Ollivier](https://github.com/maxolls) (tbd)
 
+- Updated `scrollToObject:` method in `IGListAdapter` to include a new parameter `additionalOffset` to handle shifting the final scroll position by some vertical or horizontal offset depending on the scroll direction. This allows the object to be shown at the correct position when it is scrolled to in a list with sticky headers.
+
+```objc
+// OLD
+- (void)scrollToObject:(id)object
+    supplementaryKinds:(nullable NSArray<NSString *> *)supplementaryKinds
+       scrollDirection:(UICollectionViewScrollDirection)scrollDirection
+        scrollPosition:(UICollectionViewScrollPosition)scrollPosition
+              animated:(BOOL)animated;
+
+// NEW
+- (void)scrollToObject:(id)object
+    supplementaryKinds:(nullable NSArray<NSString *> *)supplementaryKinds
+       scrollDirection:(UICollectionViewScrollDirection)scrollDirection
+        scrollPosition:(UICollectionViewScrollPosition)scrollPosition
+      additionalOffset:(CGPoint)additionalOffset
+              animated:(BOOL)animated;
+```
+
 ### Enhancements
 
 - Added `shouldSelectItemAtIndex:` to `IGListSectionController` . [dirtmelon](https://github.com/dirtmelon)

Source/IGListKit/IGListAdapter.h

@@ -248,19 +248,25 @@ NS_SWIFT_NAME(ListAdapter)
  @param supplementaryKinds The types of supplementary views in the section.
  @param scrollDirection An option indicating the direction to scroll.
  @param scrollPosition An option that specifies where the item should be positioned when scrolling finishes.
+ @param additionalOffset Additional offset amount from the scroll position.
  @param animated A flag indicating if the scrolling should be animated.
+ 
+ @note The additional offset amount is to shift the final scroll position by some horizontal or vertical amount
+ depending on the scroll direction. This is necessary when scrolling to an object on a view with sticky headers, since
+ the sticky header would otherwise cover the top portion of the object.
  */
 - (void)scrollToObject:(id)object
     supplementaryKinds:(nullable NSArray<NSString *> *)supplementaryKinds
        scrollDirection:(UICollectionViewScrollDirection)scrollDirection
         scrollPosition:(UICollectionViewScrollPosition)scrollPosition
+      additionalOffset:(CGFloat)additionalOffset
               animated:(BOOL)animated;
 
 /**
  Returns the size of a cell at the specified index path.
 
  @param indexPath The index path of the cell.
-
+å
  @return The size of the cell.
  */
 - (CGSize)sizeForItemAtIndexPath:(NSIndexPath *)indexPath;

Source/IGListKit/IGListAdapter.m

@@ -199,6 +199,7 @@ - (void)scrollToObject:(id)object
     supplementaryKinds:(NSArray<NSString *> *)supplementaryKinds
        scrollDirection:(UICollectionViewScrollDirection)scrollDirection
         scrollPosition:(UICollectionViewScrollPosition)scrollPosition
+      additionalOffset:(CGFloat)additionalOffset
               animated:(BOOL)animated {
     IGAssertMainThread();
     IGParameterAssert(object != nil);
@@ -297,6 +298,7 @@ - (void)scrollToObject:(id)object
             }
             const CGFloat maxOffsetX = collectionView.contentSize.width - collectionView.frame.size.width + contentInset.right;
             const CGFloat minOffsetX = -contentInset.left;
+            contentOffset.x += additionalOffset;
             contentOffset.x = MIN(contentOffset.x, maxOffsetX);
             contentOffset.x = MAX(contentOffset.x, minOffsetX);
             break;
@@ -324,6 +326,7 @@ - (void)scrollToObject:(id)object
             const CGFloat maxHeight = collectionView.collectionViewLayout.collectionViewContentSize.height;
             const CGFloat maxOffsetY = maxHeight - collectionView.frame.size.height + contentInset.bottom;
             const CGFloat minOffsetY = -contentInset.top;
+            contentOffset.y += additionalOffset;
             contentOffset.y = MIN(contentOffset.y, maxOffsetY);
             contentOffset.y = MAX(contentOffset.y, minOffsetY);
             break;