fix: keep native content in place when a header above the viewport resizes

__tests__/core/contentMetrics.test.ts

@@ -511,6 +511,80 @@ describe("updateContentMetrics", () => {
         }
     });
 
+    for (const os of ["ios", "android"] as const) {
+        it(`compensates native MVCP when a measured header changes above the viewport (${os})`, () => {
+            const prevPlatform = Platform.OS;
+            Platform.OS = os;
+            const requestAdjustSpy = spyOn(requestAdjustModule, "requestAdjust");
+            const ctx = createMockContext(
+                {
+                    headerSize: 60,
+                    readyToRender: true,
+                    totalSize: 1000,
+                },
+                {
+                    didContainersLayout: true,
+                    didFinishInitialScroll: true,
+                    didMeasureHeader: true,
+                    props: {
+                        data: [1],
+                        maintainVisibleContentPosition: { data: false, size: true },
+                    },
+                    scroll: 200,
+                    scrollLength: 500,
+                    totalSize: 1000,
+                },
+            );
+
+            try {
+                setHeaderSize(ctx, 120);
+
+                expect(requestAdjustSpy).toHaveBeenCalledWith(ctx, 60);
+            } finally {
+                requestAdjustSpy.mockRestore();
+                Platform.OS = prevPlatform;
+            }
+        });
+    }
+
+    it("syncs the Android clamp before compensating a header that shrinks near the end", () => {
+        const prevPlatform = Platform.OS;
+        Platform.OS = "android";
+        const requestAdjustSpy = spyOn(requestAdjustModule, "requestAdjust");
+        const ctx = createMockContext(
+            {
+                headerSize: 300,
+                readyToRender: true,
+                totalSize: 700,
+            },
+            {
+                didContainersLayout: true,
+                didFinishInitialScroll: true,
+                didMeasureHeader: true,
+                lastLayout: { height: 500, width: 300, x: 0, y: 0 },
+                props: {
+                    data: [1],
+                    maintainVisibleContentPosition: { data: false, size: true },
+                },
+                scroll: 450,
+                scrollLength: 500,
+                totalSize: 700,
+            },
+        );
+
+        try {
+            setHeaderSize(ctx, 100);
+
+            // The header and rows fill 1000 points. The 200-point shrink leaves 800, so Android clamps the offset to 300 before the
+            // adjustment lands. Only the remaining 50 points are adjusted, which ends at the kept position, 250.
+            expect(requestAdjustSpy).toHaveBeenCalledWith(ctx, -50);
+            expect(ctx.state.scroll).toBe(250);
+        } finally {
+            requestAdjustSpy.mockRestore();
+            Platform.OS = prevPlatform;
+        }
+    });
+
     it("does not compensate the initial web MVCP header measurement", () => {
         const prevPlatform = Platform.OS;
         Platform.OS = "web";

src/core/clampScrollOffset.ts

@@ -3,6 +3,8 @@ import { getContentSize } from "@/state/getContentSize";
 import type { StateContext } from "@/state/state";
 import type { ScrollTarget } from "@/types.internal";
 
+const ANDROID_SHRINK_CLAMP_EPSILON = 0.1;
+
 export function getContentInsetTop(ctx: StateContext) {
     const state = ctx.state;
     if (Platform.OS !== "ios" || state.props.horizontal) {
@@ -16,6 +18,27 @@ export function getScrollRangeEnd(ctx: StateContext, contentSize = getContentSiz
     return Math.max(0 - getContentInsetTop(ctx), contentSize - ctx.state.scrollLength);
 }
 
+/**
+ * Android clamps the scroll offset when content shrinks, before native MVCP applies an anchor move. Syncs the JS
+ * scroll state to that clamp and returns the part of the adjustment the clamp did not already apply.
+ */
+export function getAdjustAfterAndroidShrinkClamp(ctx: StateContext, positionDiff: number) {
+    const state = ctx.state;
+    if (Platform.OS !== "android") {
+        return positionDiff;
+    }
+
+    const maxScroll = getScrollRangeEnd(ctx);
+    if (state.scroll <= maxScroll + ANDROID_SHRINK_CLAMP_EPSILON) {
+        return positionDiff;
+    }
+
+    const targetScroll = Math.max(0, Math.min(state.scroll + positionDiff, maxScroll));
+    state.scroll = maxScroll;
+    state.scrollPending = maxScroll;
+    return targetScroll - maxScroll;
+}
+
 export function clampScrollOffset(ctx: StateContext, offset: number, scrollTarget?: Partial<ScrollTarget>) {
     const state = ctx.state;
     const minOffset = 0 - getContentInsetTop(ctx);

src/core/mvcp.ts

@@ -1,6 +1,6 @@
 import { IsNewArchitecture } from "@/constants-platform";
 import { getViewPositionOffset } from "@/core/calculateOffsetWithOffsetPosition";
-import { getContentInsetTop, getScrollRangeEnd } from "@/core/clampScrollOffset";
+import { getAdjustAfterAndroidShrinkClamp, getContentInsetTop, getScrollRangeEnd } from "@/core/clampScrollOffset";
 import { shouldMaintainScrollAtEndOnDataChange } from "@/core/shouldMaintainScrollAtEndOnDataChange";
 import { Platform } from "@/platform/Platform";
 import { getContentInsetEnd } from "@/state/getContentInsetEnd";
@@ -465,15 +465,8 @@ export function prepareMVCP(
                 positionDiff,
             });
 
-            if (Platform.OS === "android" && !dataChanged && scrollTarget === undefined) {
-                const maxScroll = getScrollRangeEnd(ctx);
-                if (state.scroll > maxScroll + MVCP_POSITION_EPSILON) {
-                    // Android clamps shrinking content before native MVCP applies the anchor delta.
-                    const targetScroll = Math.max(0, Math.min(state.scroll + positionDiff, maxScroll));
-                    state.scroll = maxScroll;
-                    state.scrollPending = maxScroll;
-                    positionDiff = targetScroll - maxScroll;
-                }
+            if (!dataChanged && scrollTarget === undefined) {
+                positionDiff = getAdjustAfterAndroidShrinkClamp(ctx, positionDiff);
             }
 
             if (

src/core/updateContentMetrics.ts

@@ -1,3 +1,4 @@
+import { getAdjustAfterAndroidShrinkClamp } from "@/core/clampScrollOffset";
 import { Platform } from "@/platform/Platform";
 import { peek$, type StateContext, set$ } from "@/state/state";
 import type { Insets } from "@/types.base";
@@ -26,7 +27,6 @@ function shouldAdjustForHeaderSizeChange(ctx: StateContext, previousHeaderSize:
     const previousHeaderEnd = (leadingPadding || 0) + previousHeaderSize;
 
     return (
-        Platform.OS === "web" &&
         props.maintainVisibleContentPosition.size &&
         didContainersLayout &&
         didFinishInitialScroll &&
@@ -49,8 +49,9 @@ export function setHeaderSize(ctx: StateContext, size: number) {
         // Do not compensate the initial measurement from no known header.
         // If a zero/estimated header was already recorded, this layout change
         // is a real content shift above the viewport and should preserve MVCP.
+        // Native MVCP anchors to an absolutely positioned view that a header resize does not move, so move it here.
         if (hasMeasuredOrEstimatedHeaderBaseline && shouldAdjustForHeaderSizeChange(ctx, previousHeaderSize, size)) {
-            requestAdjust(ctx, size - previousHeaderSize);
+            requestAdjust(ctx, getAdjustAfterAndroidShrinkClamp(ctx, size - previousHeaderSize));
         }
     }