Address review feedback on wrap viewport anchoring

Author: benvinegar

src/ui/components/panes/DiffPane.tsx

@@ -10,7 +10,11 @@ import {
 } from "react";
 import type { DiffFile, LayoutMode } from "../../../core/types";
 import type { VisibleAgentNote } from "../../lib/agentAnnotations";
-import { measureDiffSectionMetrics, type DiffSectionMetrics } from "../../lib/sectionHeights";
+import {
+  measureDiffSectionMetrics,
+  type DiffSectionMetrics,
+  type DiffSectionRowMetric,
+} from "../../lib/sectionHeights";
 import { diffHunkId, diffSectionId } from "../../lib/ids";
 import type { AppTheme } from "../../themes";
 import { DiffSection } from "./DiffSection";
@@ -25,6 +29,26 @@ interface ViewportRowAnchor {
   rowOffsetWithin: number;
 }
 
+function binarySearchRowMetric(rowMetrics: DiffSectionRowMetric[], relativeTop: number) {
+  let low = 0;
+  let high = rowMetrics.length - 1;
+
+  while (low <= high) {
+    const mid = (low + high) >>> 1;
+    const rowMetric = rowMetrics[mid]!;
+
+    if (relativeTop < rowMetric.offset) {
+      high = mid - 1;
+    } else if (relativeTop >= rowMetric.offset + rowMetric.height) {
+      low = mid + 1;
+    } else {
+      return rowMetric;
+    }
+  }
+
+  return undefined;
+}
+
 function findViewportRowAnchor(
   files: DiffFile[],
   sectionMetrics: DiffSectionMetrics[],
@@ -44,10 +68,7 @@ function findViewportRowAnchor(
     const relativeTop = scrollTop - bodyTop;
 
     if (relativeTop >= 0 && relativeTop < bodyHeight && metrics) {
-      const rowMetric = metrics.rowMetrics.find(
-        (candidate) =>
-          relativeTop >= candidate.offset && relativeTop < candidate.offset + candidate.height,
-      );
+      const rowMetric = binarySearchRowMetric(metrics.rowMetrics, relativeTop);
       if (rowMetric) {
         return {
           fileId: files[index]!.id,
@@ -407,11 +428,10 @@ export function DiffPane({
     const previousFiles = previousFilesRef.current;
 
     if (wrapChanged && previousSectionMetrics && previousFiles.length > 0) {
-      const previousScrollTop = Math.max(
-        wrapToggleScrollTop ?? 0,
-        prevScrollTopRef.current,
-        scrollViewport.top,
-      );
+      const previousScrollTop =
+        wrapToggleScrollTop != null
+          ? wrapToggleScrollTop
+          : Math.max(prevScrollTopRef.current, scrollViewport.top);
       const anchor = findViewportRowAnchor(
         previousFiles,
         previousSectionMetrics,
@@ -425,6 +445,8 @@ export function DiffPane({
 
         restoreViewportAnchor();
         suppressNextSelectionAutoScrollRef.current = true;
+        // Retry across a couple of repaint cycles so the restored top-row anchor sticks
+        // after wrapped row heights and viewport culling settle.
         const retryDelays = [0, 16, 48];
         const timeouts = retryDelays.map((delay) => setTimeout(restoreViewportAnchor, delay));
 
@@ -470,7 +492,8 @@ export function DiffPane({
       scrollBox.scrollChildIntoView(selectedAnchorId);
     };
 
-    // Run after this pane renders the selected section/hunk, then retry briefly while layout settles.
+    // Run after this pane renders the selected section/hunk, then retry briefly while layout
+    // settles across a couple of repaint cycles.
     scrollSelectionIntoView();
     const retryDelays = [0, 16, 48];
     const timeouts = retryDelays.map((delay) => setTimeout(scrollSelectionIntoView, delay));