fix: recompute full layout range on initial scroll adjustment

src/__tests__/initialScrollIndex2307.test.tsx

@@ -0,0 +1,49 @@
+import React from "react";
+import { Text } from "react-native";
+import "@quilted/react-testing/matchers";
+import { render } from "@quilted/react-testing";
+
+import { FlashList } from "..";
+
+// Items are 300px tall, which is above the 200px default the layout manager
+// seeds unmeasured items with. That gap is what issue #2307 depends on.
+jest.mock("../recyclerview/utils/measureLayout", () => {
+  const originalModule = jest.requireActual(
+    "../recyclerview/utils/measureLayout"
+  );
+  return {
+    ...originalModule,
+    measureParentSize: jest
+      .fn()
+      .mockImplementation(() => ({ width: 400, height: 900 })),
+    measureFirstChildLayout: jest
+      .fn()
+      .mockImplementation(() => ({ x: 0, y: 0, width: 400, height: 900 })),
+    measureItemLayout: jest
+      .fn()
+      .mockImplementation(() => ({ x: 0, y: 0, width: 400, height: 300 })),
+  };
+});
+
+describe("initialScrollIndex with items taller than the default estimate", () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+    jest.useFakeTimers();
+  });
+
+  it("opens on the requested item rather than one far past it", () => {
+    const result = render(
+      <FlashList
+        data={Array.from({ length: 600 }, (_, index) => index)}
+        keyExtractor={(item) => String(item)}
+        initialScrollIndex={250}
+        renderItem={({ item }) => <Text>{item}</Text>}
+        overrideProps={{ initialDrawBatchSize: 1 }}
+        drawDistance={0}
+      />
+    );
+
+    expect(result).toContainReactComponent(Text, { children: 250 });
+    expect(result).not.toContainReactComponent(Text, { children: 333 });
+  });
+});

src/recyclerview/RecyclerViewManager.ts

@@ -385,7 +385,15 @@ export class RecyclerViewManager<T> {
       // re-estimate unmeasured items with an updated average height, changing
       // the target item's position. Reading before recompute would capture a
       // stale offset, causing the wrong items to be rendered.
-      this.layoutManager.recomputeLayouts(0, initialScrollIndex);
+      //
+      // This has to run to the end of the list rather than stopping at
+      // initialScrollIndex. Items are positioned from their predecessor, so
+      // recomputing only the prefix moves the target item without moving the
+      // items after it, and the layout array is left out of order at that
+      // boundary. getVisibleLayouts binary searches that array and documents
+      // that it assumes it is sorted, so once it is not, the search can return
+      // an item nowhere near the one requested.
+      this.layoutManager.recomputeLayouts(0, this.getDataLength() - 1);
       const initialItemLayout =
         this.layoutManager.getLayout(initialScrollIndex);
       const initialItemOffset = this.propsRef.horizontal