fix: consistent diff stats between preview badge and DiffViewer

- Export computeDiffLineStats from DiffViewer — uses the same LCS/semantic
  diff algorithm so stats always match the rendered diff
- Remove custom computeDiffStats (set-based) that produced different counts
- Add quickStats from toolInput.content for Write tool — shows +N
  immediately before IPC completes (no need to expand first)
- diffStats (accurate, from IPC) replaces quickStats once available

src/renderer/components/chat/viewers/DiffViewer.tsx

@@ -149,6 +149,19 @@ function generateDiff(oldLines: string[], newLines: string[]): DiffLine[] {
   return result;
 }
 
+/**
+ * Computes diff statistics from old/new strings using the same algorithm as DiffViewer.
+ */
+export function computeDiffLineStats(
+  oldString: string,
+  newString: string
+): { added: number; removed: number } {
+  const oldLines = oldString.split(/\r?\n/);
+  const newLines = newString.split(/\r?\n/);
+  const diffLines = generateDiff(oldLines, newLines);
+  return computeStats(diffLines);
+}
+
 /**
  * Computes diff statistics.
  */

src/renderer/components/team/ToolApprovalDiffPreview.tsx

@@ -1,6 +1,6 @@
 import React, { useMemo, useState } from 'react';
 
-import { DiffViewer } from '@renderer/components/chat/viewers/DiffViewer';
+import { computeDiffLineStats, DiffViewer } from '@renderer/components/chat/viewers/DiffViewer';
 import { useToolApprovalDiff } from '@renderer/hooks/useToolApprovalDiff';
 import { AlertTriangle, ChevronDown, ChevronRight, FileDiff, Loader2 } from 'lucide-react';
 
@@ -35,51 +35,18 @@ function saveExpandedPref(value: boolean): void {
 }
 
 // =============================================================================
-// Diff stats helper
+// Quick stats from toolInput (before IPC, for Write tool)
 // =============================================================================
 
-function computeDiffStats(
-  oldString: string,
-  newString: string
-): { added: number; removed: number } {
-  const oldLines = oldString.split(/\r?\n/);
-  const newLines = newString.split(/\r?\n/);
-  // Simple line-count based stats (matches DiffViewer's own count)
-  const maxLen = Math.max(oldLines.length, newLines.length);
-  let added = 0;
-  let removed = 0;
-  // Count lines that differ
-  if (oldString === '' && newString !== '') {
-    added = newLines.length;
-  } else if (newString === '' && oldString !== '') {
-    removed = oldLines.length;
-  } else {
-    // Diff-based: count added/removed from line diff
-    const oldSet = new Map<string, number>();
-    for (const line of oldLines) {
-      oldSet.set(line, (oldSet.get(line) ?? 0) + 1);
-    }
-    const newSet = new Map<string, number>();
-    for (const line of newLines) {
-      newSet.set(line, (newSet.get(line) ?? 0) + 1);
-    }
-    // Lines in new but not in old
-    for (const [line, count] of newSet) {
-      const oldCount = oldSet.get(line) ?? 0;
-      if (count > oldCount) added += count - oldCount;
-    }
-    // Lines in old but not in new
-    for (const [line, count] of oldSet) {
-      const newCount = newSet.get(line) ?? 0;
-      if (count > newCount) removed += count - newCount;
-    }
-    // Ensure at least something shows if strings differ but stats are 0
-    if (added === 0 && removed === 0 && oldString !== newString) {
-      added = Math.max(0, newLines.length - maxLen);
-      removed = Math.max(0, oldLines.length - maxLen);
-    }
+function computeQuickStats(
+  toolName: string,
+  toolInput: Record<string, unknown>
+): { added: number; removed: number } | null {
+  if (toolName === 'Write' && typeof toolInput.content === 'string') {
+    const lines = toolInput.content.split(/\r?\n/).length;
+    return { added: lines, removed: 0 };
   }
-  return { added, removed };
+  return null;
 }
 
 // =============================================================================
@@ -95,12 +62,23 @@ export const ToolApprovalDiffPreview: React.FC<ToolApprovalDiffPreviewProps> = (
   const [expanded, setExpanded] = useState(loadExpandedPref);
   const diff = useToolApprovalDiff(toolName, toolInput, requestId, expanded);
 
-  const stats = useMemo(() => {
+  // Stats from actual diff data (after IPC), using the same algorithm as DiffViewer
+  const diffStats = useMemo(() => {
     if (!diff.hasDiff || diff.loading || diff.isBinary || diff.error) return null;
     if (!diff.oldString && !diff.newString) return null;
-    return computeDiffStats(diff.oldString, diff.newString);
+    return computeDiffLineStats(diff.oldString, diff.newString);
   }, [diff.hasDiff, diff.loading, diff.isBinary, diff.error, diff.oldString, diff.newString]);
 
+  // Quick stats from toolInput (available immediately, before diff loads)
+  const quickStats = useMemo(
+    () => computeQuickStats(toolName, toolInput),
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- toolInput identity tied to requestId
+    [toolName, requestId]
+  );
+
+  // Prefer accurate diffStats when available, fall back to quickStats
+  const stats = diffStats ?? quickStats;
+
   if (!DIFF_TOOLS.has(toolName)) return null;
 
   const toggleExpanded = (): void => {