Perf regression bugfix

lib/core/getStartStopIndices.test.ts

@@ -291,6 +291,41 @@ test("initial variable-size lookup only measures the viewport", () => {
   expect(measurements).toBe(25);
 });
 
+test.each([0, 25])(
+  "deep variable-size lookup with %i cached rows only measures through the viewport",
+  (cachedRowCount) => {
+    let measurements = 0;
+    const cachedBounds = createCachedBounds({
+      itemCount: 100000,
+      itemProps: {},
+      itemSize: (index) => {
+        measurements++;
+        return index % 2 === 0 ? 10 : 30;
+      }
+    });
+    if (cachedRowCount > 0) {
+      cachedBounds.get(cachedRowCount - 1);
+    }
+    measurements = 0;
+
+    const result = getStartStopIndices({
+      cachedBounds,
+      containerScrollOffset: 50000,
+      containerSize: 80,
+      itemCount: 100000,
+      overscanCount: 3
+    });
+
+    expect(result).toEqual({
+      startIndexVisible: 2500,
+      stopIndexVisible: 2503,
+      startIndexOverscan: 2497,
+      stopIndexOverscan: 2506
+    });
+    expect(measurements).toBe(2504 - cachedRowCount);
+  }
+);
+
 test("matches a linear lookup across boundaries, zero sizes and scroll directions", () => {
   for (const sizes of [
     [],

lib/core/getStartStopIndices.ts

@@ -46,18 +46,9 @@ export function getStartStopIndices({
       )
     );
   } else {
-    // Search the measured prefix first. Grow the search exponentially only
-    // when scrolling beyond it, so an initial render doesn't measure the list.
+    // Binary search the measured prefix
     let low = 0;
     let high = Math.min(maxIndex, Math.max(0, cachedBounds.size - 1));
-    let step = 1;
-    while (high < maxIndex) {
-      const bounds = cachedBounds.get(high);
-      if (bounds.scrollOffset + bounds.size > containerScrollOffset) break;
-      low = high + 1;
-      high = Math.min(maxIndex, high + step);
-      step *= 2;
-    }
 
     while (low < high) {
       const middle = Math.floor((low + high) / 2);
@@ -69,6 +60,13 @@ export function getStartStopIndices({
       }
     }
 
+    // Extend sequentially beyond the measured prefix to avoid evaluating item sizes past the viewport
+    while (low < maxIndex) {
+      const bounds = cachedBounds.get(low);
+      if (bounds.scrollOffset + bounds.size > containerScrollOffset) break;
+      low++;
+    }
+
     startIndexVisible = low;
     stopIndexVisible = low;
     while (stopIndexVisible < maxIndex) {