Fix #12

CHANGELOG.md

@@ -3,6 +3,7 @@ All notable changes to this project will be documented in this file.
 
 ## 2.0.3
 
+* Fix #12 Scroll doesn't reset when switching the tabs while scrolled to top
 * Fix the bottom padding on `MaterialTabsScroll` in iOS 18
 
 ## 2.0.2

Sources/SwiftUIMaterialTabs/Internal/HeaderModel.swift

@@ -73,24 +73,42 @@ class HeaderModel<Tab>: ObservableObject where Tab: Hashable {
 
     // MARK: - Height
 
-
-
     // MARK: - Scroll tracking
 
-    func scrolled(tab: Tab, offset: CGFloat, deltaOffset: CGFloat) {
+    /// Adjust the header offset as the scroll view's offset changes. As discussed elsewhere in this library, the header view itself doesn't shrink—instead its vertical
+    /// position is shifted up until it reaches a maximum corresponding to a fully collapsed header state. Scroll effects can be applied to subviews within the
+    /// header to give the appearance of shrinking, among other things.
+    ///
+    /// In the basic case, the header offset matches the scroll view up calculated max offset. However, in a multi-tab environment, scrolling on another tab
+    /// can change the header offset, introducing edge cases that need to be handled.
+    func scrolled(tab: Tab, contentOffset: CGFloat, deltaContentOffset: CGFloat) {
         guard tab == state.headerContext.selectedTab else { return }
-        //print("tab=\(tab), offset=\(offset), deltaOffset=\(deltaOffset)")
-        state.headerContext.contentOffset = offset        
-        // Any time the offset is less than the max offset, the header offset exactly tracks the offset.
-        if offset < state.headerContext.maxOffset {
-            state.headerContext.offset = offset
-        }
-        // However, for greater offsets, the header offset only gets adjusted for positive changes in the offset.
-        // Once we scroll too far, the header offset hits the limit, so we can't just track the offset. Instead, we
-        // use the change in offset.
-        else if deltaOffset > 0 {
-            let unconstrainedOffset = state.headerContext.offset + deltaOffset
-            state.headerContext.offset = min(unconstrainedOffset, state.headerContext.maxOffset)
+        state.headerContext.contentOffset = contentOffset
+        // When scrolling down (a.k.a. swiping up), the header offset matches the scroll view until it reaches the
+        // max offset, at which point it is fully collapsed.
+        if deltaContentOffset > 0 {
+            // If the scroll view offset is less than the max offset, then the scroll and header offsets should
+            // match.
+            if contentOffset < state.headerContext.maxOffset {
+                state.headerContext.offset = contentOffset
+            }
+            // However, if the scroll view is past the max offset, the header must move by the same amount until
+            // it reaches the max offset.
+            else {
+                state.headerContext.offset = min(
+                    state.headerContext.offset + deltaContentOffset,
+                    state.headerContext.maxOffset
+                )
+            }
+        // When scrolling up (a.k.a. swiping down), the header offset remains fixed unless it needs to change to
+        // prevent the header from separating from the scroll view content. This threshold is reached when the
+        // top of the scroll view reaches the bottom of the header.
+        } else {
+            // If the scroll view's offset is less than the header's offset, the header must match the scroll view
+            // offset to avoid separation.
+            if contentOffset < state.headerContext.offset {
+                state.headerContext.offset = contentOffset
+            }
         }
     }
 }

Sources/SwiftUIMaterialTabs/Internal/ScrollModel.swift

@@ -21,7 +21,7 @@ class ScrollModel<Item, Tab>: ObservableObject where Item: Hashable, Tab: Hashab
         // This is how we're detecting programatic scroll for lack of a better idea. We don't want to report
         // the programatic sync of content offset with the header because it could result in the header moving.
         guard scrollItem != reservedItem else { return }
-        headerModel?.scrolled(tab: tab, offset: contentOffset, deltaOffset: deltaOffset)
+        headerModel?.scrolled(tab: tab, contentOffset: contentOffset, deltaContentOffset: deltaOffset)
     }
 
     func appeared(headerModel: HeaderModel<Tab>?) {

Sources/SwiftUIMaterialTabs/StickyHeader/StickyHeaderScroll.swift

@@ -59,7 +59,11 @@ public struct StickyHeaderScroll<Content, Item>: View where Content: View, Item:
                         }
                     }
                     .onPreferenceChange(ScrollOffsetPreferenceKey.self) { offset in
-                        headerModel.scrolled(tab: .none, offset: -offset, deltaOffset: -(offset - contentOffset))
+                        headerModel.scrolled(
+                            tab: .none,
+                            contentOffset: -offset,
+                            deltaContentOffset: -(offset - contentOffset)
+                        )
                         contentOffset = -offset
                     }
                 content(