From 432a9dcaba2dfadb3990f02fb0778aab7214c035 Mon Sep 17 00:00:00 2001
From: Ian Luo <iluo@squareup.com>
Date: Fri, 12 Mar 2021 11:36:38 -0800
Subject: [PATCH] Fix `scrollToItem` appearing underneath sticky header

When a `top` scroll position is used, the item will appear underneath the sticky section header.
Now, we adjust the frame and use a manual content offset when sticky headers are enabled.
When sticky headers are disabled or scrolling to a different position, we call the existing `scrollToItem` function on UICollectionView.
---
 ListableUI/Sources/ListView/ListView.swift | 35 ++++++++++++++++------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/ListableUI/Sources/ListView/ListView.swift b/ListableUI/Sources/ListView/ListView.swift
index b14d8b5ea..564dc740c 100644
--- a/ListableUI/Sources/ListView/ListView.swift
+++ b/ListableUI/Sources/ListView/ListView.swift
@@ -424,27 +424,44 @@ public final class ListView : UIView, KeyboardObserverDelegate
         }
         
         return self.preparePresentationStateForScroll(to: toIndexPath) {
-                        
-            let isAlreadyVisible: Bool = {
-                let frame = self.collectionViewLayout.frameForItem(at: toIndexPath)
+            let itemFrame = self.collectionViewLayout.frameForItem(at: toIndexPath)
 
-                return self.collectionView.contentFrame.contains(frame)
-            }()
+            let isAlreadyVisible = self.collectionView.contentFrame.contains(itemFrame)
 
             // If the item is already visible and that's good enough, return.
 
             if isAlreadyVisible && position.ifAlreadyVisible == .doNothing {
                 return
             }
-            
-            animation.perform(
-                animations: {
+
+            let scroll: () -> Void = {
+                let sectionHeader = self.collectionViewLayout.layout.content.sections[toIndexPath.section].header
+
+                // Prevent the item from appearing underneath a sticky section header.
+
+                if sectionHeader.isPopulated,
+                   self.collectionViewLayout.layout.stickySectionHeaders,
+                   position.position == .top {
+
+                    let itemFrameAdjustedForStickyHeaders = CGRect(
+                        x: itemFrame.minX,
+                        y: itemFrame.minY - sectionHeader.size.height,
+                        width: itemFrame.width,
+                        height: itemFrame.height
+                    )
+                    self.performScroll(to: itemFrameAdjustedForStickyHeaders, scrollPosition: position)
+
+                } else {
                     self.collectionView.scrollToItem(
                         at: toIndexPath,
                         at: position.position.UICollectionViewScrollPosition,
                         animated: false
                     )
-                },
+                }
+            }
+            
+            animation.perform(
+                animations: scroll,
                 completion: completion
             )
         }