Merge pull request Expensify#88923 from callstack-internal/fix/keep-scrollanchor-mounted

fix(iOS): keep ScrollAnchor mounted to prevent scroll reset when MVCP toggles

Author: arosiclair

patches/@shopify/flash-list/@shopify+flash-list+2.3.0+007+fix-scroll-anchor-unmount-on-ios.patch

@@ -0,0 +1,80 @@
+diff --git a/node_modules/@shopify/flash-list/dist/recyclerview/RecyclerView.js b/node_modules/@shopify/flash-list/dist/recyclerview/RecyclerView.js
+index ee42f63..4e8d8c0 100644
+--- a/node_modules/@shopify/flash-list/dist/recyclerview/RecyclerView.js
++++ b/node_modules/@shopify/flash-list/dist/recyclerview/RecyclerView.js
+@@ -380,16 +380,15 @@ const RecyclerViewComponent = (props, ref) => {
+         }
+         return onScrollHandler;
+     }, [onScrollHandler, scrollY, stickyHeaders, stickyHeaderUseNativeDriver]);
+-    const shouldMaintainVisibleContentPosition = recyclerViewManager.shouldMaintainVisibleContentPosition();
+     const maintainVisibleContentPositionInternal = useMemo(() => {
+-        if (shouldMaintainVisibleContentPosition) {
++        if (maintainVisibleContentPosition != null) {
+             return {
+                 ...maintainVisibleContentPosition,
+                 minIndexForVisible: 0,
+             };
+         }
+         return undefined;
+-    }, [maintainVisibleContentPosition, shouldMaintainVisibleContentPosition]);
++    }, [maintainVisibleContentPosition]);
+     const shouldRenderFromBottom = recyclerViewManager.getDataLength() > 0 &&
+         ((_d = maintainVisibleContentPosition === null || maintainVisibleContentPosition === void 0 ? void 0 : maintainVisibleContentPosition.startRenderingFromBottom) !== null && _d !== void 0 ? _d : false);
+     // Create view for measuring bounded size
+@@ -401,11 +399,11 @@ const RecyclerViewComponent = (props, ref) => {
+             }, ref: firstChildViewRef }));
+     }, [horizontal, stickyHeaderOffset]);
+     const scrollAnchor = useMemo(() => {
+-        if (shouldMaintainVisibleContentPosition) {
++        if (maintainVisibleContentPosition != null) {
+             return (React.createElement(ScrollAnchor, { horizontal: Boolean(horizontal), scrollAnchorRef: scrollAnchorRef }));
+         }
+         return null;
+-    }, [horizontal, shouldMaintainVisibleContentPosition]);
++    }, [horizontal, maintainVisibleContentPosition]);
+     // console.log("render", recyclerViewManager.getRenderStack());
+     // Render the main RecyclerView structure
+     return (React.createElement(RecyclerViewContextProvider, { value: recyclerViewContext },
+diff --git a/node_modules/@shopify/flash-list/src/recyclerview/RecyclerView.tsx b/node_modules/@shopify/flash-list/src/recyclerview/RecyclerView.tsx
+index b2bd67a..d4bf02d 100644
+--- a/node_modules/@shopify/flash-list/src/recyclerview/RecyclerView.tsx
++++ b/node_modules/@shopify/flash-list/src/recyclerview/RecyclerView.tsx
+@@ -572,18 +572,15 @@ const RecyclerViewComponent = <T,>(
+     return onScrollHandler;
+   }, [onScrollHandler, scrollY, stickyHeaders, stickyHeaderUseNativeDriver]);
+ 
+-  const shouldMaintainVisibleContentPosition =
+-    recyclerViewManager.shouldMaintainVisibleContentPosition();
+-
+   const maintainVisibleContentPositionInternal = useMemo(() => {
+-    if (shouldMaintainVisibleContentPosition) {
++    if (maintainVisibleContentPosition != null) {
+       return {
+         ...maintainVisibleContentPosition,
+         minIndexForVisible: 0,
+       };
+     }
+     return undefined;
+-  }, [maintainVisibleContentPosition, shouldMaintainVisibleContentPosition]);
++  }, [maintainVisibleContentPosition]);
+ 
+   const shouldRenderFromBottom =
+     recyclerViewManager.getDataLength() > 0 &&
+@@ -604,7 +600,7 @@ const RecyclerViewComponent = <T,>(
+   }, [horizontal, stickyHeaderOffset]);
+ 
+   const scrollAnchor = useMemo(() => {
+-    if (shouldMaintainVisibleContentPosition) {
++    if (maintainVisibleContentPosition != null) {
+       return (
+         <ScrollAnchor
+           horizontal={Boolean(horizontal)}
+@@ -613,7 +609,7 @@ const RecyclerViewComponent = <T,>(
+       );
+     }
+     return null;
+-  }, [horizontal, shouldMaintainVisibleContentPosition]);
++  }, [horizontal, maintainVisibleContentPosition]);
+ 
+   // console.log("render", recyclerViewManager.getRenderStack());
+ 

patches/@shopify/flash-list/details.md

@@ -48,3 +48,11 @@
 - Upstream PR/issue: TBD
 - E/App issue: https://github.com/Expensify/App/issues/33725
 - PR introducing patch: https://github.com/Expensify/App/pull/85114
+
+### [@shopify+flash-list+2.3.0+007+fix-scroll-anchor-unmount-on-ios.patch](@shopify+flash-list+2.3.0+007+fix-scroll-anchor-unmount-on-ios.patch)
+
+- Reason: Fixes a scroll position reset on iOS when `maintainVisibleContentPosition.disabled` toggles from `true` to `false` (e.g. when `shouldMaintainVisibleContentPosition` changes based on scroll offset). Root cause: `ScrollAnchor` was conditionally rendered based on `shouldMaintainVisibleContentPosition()`. When MVCP was disabled, the anchor unmounted, which made the native Fabric `_firstVisibleView` weak-ref become nil. When MVCP was re-enabled, the anchor remounted at `top: 1,000,000` (its initial position), but `_prevFirstVisibleFrame` was stale at `1,000,000 + X` from the prior anchor instance. `_adjustForMaintainVisibleContentPosition` then computed `deltaY = 0 - (1,000,000 + X)` — a massive negative offset — causing the list to jump to the start. The fix decouples anchor lifetime from the `disabled` flag: `ScrollAnchor` is now always mounted (and `maintainVisibleContentPositionInternal` always non-null) whenever `maintainVisibleContentPosition` prop is defined. The `disabled` flag continues to gate JS-level `scrollBy` corrections in `applyOffsetCorrection` (via `shouldMaintainVisibleContentPosition()`), so the anchor stays in place when MVCP is logically off — the native side always has a live `_firstVisibleView` and a fresh `_prevFirstVisibleFrame` to diff against.
+- Files changed: Both `src/recyclerview/RecyclerView.tsx` and `dist/recyclerview/RecyclerView.js`.
+- Upstream PR/issue: TBD
+- E/App issue: https://github.com/Expensify/App/issues/33725
+- PR introducing patch: TBD