fix: apply a deferred item-size MVCP correction in the next position pass

__tests__/core/calculateItemsInView.test.ts

@@ -326,6 +326,22 @@ describe("calculateItemsInView", () => {
         }
     });
 
+    it("applies a deferred item-size MVCP correction in the next pass that repositions items", () => {
+        setupFixedSizeItems(30, 10);
+        const deferredPass = mock(() => {});
+        mockState.scheduledWork.frame(deferredPass, "mvcpRecalculate");
+        const prepareMVCPSpy = spyOn(mvcpModule, "prepareMVCP").mockImplementation(() => undefined);
+        try {
+            // A scroll-driven pass runs before the deferred frame.
+            calculateItemsInView(mockCtx, { scrollVelocity: 0 });
+
+            expect(prepareMVCPSpy).toHaveBeenCalledWith(mockCtx, undefined, "item-size");
+            expect(mockState.scheduledWork.has("mvcpRecalculate")).toBe(false);
+        } finally {
+            prepareMVCPSpy.mockRestore();
+        }
+    });
+
     it("limits cold assignments to the current buffer and header-corrected animated target union", () => {
         setupFixedSizeItems(120, 64);
         mockState.props.estimatedItemSize = 64;

src/core/calculateItemsInView.ts

@@ -345,7 +345,8 @@ export function calculateItemsInView(
         const stickyHeaderIndicesArr = state.props.stickyHeaderIndicesArr || [];
         const stickyHeaderIndicesSet = state.props.stickyHeaderIndicesSet || new Set<number>();
         const drawDistance = getEffectiveDrawDistance(ctx, params.drawDistanceMode);
-        const { dataChanged, doMVCP, forceFullItemPositions, mvcpAdjustmentSource } = params;
+        const { dataChanged, forceFullItemPositions } = params;
+        let { doMVCP, mvcpAdjustmentSource } = params;
         const bootstrapInitialScrollState =
             state.initialScrollSession?.kind === "bootstrap" ? state.initialScrollSession.bootstrap : undefined;
         const suppressInitialScrollSideEffects = !!bootstrapInitialScrollState;
@@ -503,6 +504,13 @@ export function calculateItemsInView(
             }
         }
 
+        // A deferred item-size correction must land in any pass that repositions items, or they move without it.
+        if (!doMVCP && state.scheduledWork.has("mvcpRecalculate")) {
+            state.scheduledWork.cancel("mvcpRecalculate");
+            doMVCP = true;
+            mvcpAdjustmentSource = "item-size";
+        }
+
         ////// Update item positions and do MVCP
         // Handle maintainVisibleContentPosition adjustment early
         const checkMVCP =