fix: compute first changed diff line from patch hunks

Uses hunk contents to find the first modified line instead of the hunk start
Handles added, removed, and binary-only patches more accurately
Adds tests for patch parsing edge cases

packages/ui/src/components/views/DiffView.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, test } from 'bun:test';
+
+import { getFirstChangedModifiedLineFromPatch } from './diffPatchUtils';
+
+describe('getFirstChangedModifiedLineFromPatch', () => {
+  test('returns the first added line instead of the hunk context start', () => {
+    expect(getFirstChangedModifiedLineFromPatch(`diff --git a/src/file.ts b/src/file.ts
+@@ -56,10 +56,11 @@
+ unchanged 58
+ unchanged 59
+ unchanged 60
++changed 61
+ unchanged 62`)).toBe(59);
+  });
+
+  test('returns the following modified line for deletion-only hunks', () => {
+    expect(getFirstChangedModifiedLineFromPatch(`@@ -10,4 +10,3 @@
+ context
+-removed
+ after`)).toBe(11);
+  });
+
+  test('returns null when the patch has no hunk change lines', () => {
+    expect(getFirstChangedModifiedLineFromPatch('Binary files a/image.png and b/image.png differ')).toBeNull();
+  });
+});

packages/ui/src/components/views/DiffView.tsx

@@ -39,6 +39,7 @@ import { fileDiffFromPatch } from '@/lib/diff/patchFileDiff';
 import { isVSCodeRuntime } from '@/lib/desktop';
 import { startReviewFlow } from '@/lib/reviewFlow';
 import { useSessionUIStore } from '@/sync/session-ui-store';
+import { getFirstChangedModifiedLineFromPatch } from './diffPatchUtils';
 import type { FileDiffMetadata } from '@pierre/diffs';
 
 // Minimum width for side-by-side diff view (px)
@@ -160,24 +161,6 @@ const getFirstChangedModifiedLine = (original: string, modified: string): number
     return 1;
 };
 
-const getFirstVisibleModifiedLineFromPatch = (patch: string): number | null => {
-    if (!patch) {
-        return null;
-    }
-
-    const match = patch.match(/@@\s*-\d+(?:,\d+)?\s+\+(\d+)(?:,\d+)?\s*@@/m);
-    if (!match) {
-        return null;
-    }
-
-    const parsed = Number.parseInt(match[1], 10);
-    if (!Number.isFinite(parsed) || parsed < 1) {
-        return null;
-    }
-
-    return parsed;
-};
-
 const isBinaryPatch = (patch: string): boolean =>
     /^Binary files .+ differ$/m.test(patch) || /^GIT binary patch$/m.test(patch);
 
@@ -1422,7 +1405,9 @@ export const DiffView: React.FC<DiffViewProps> = ({
         try {
             let targetLine: number | null = null;
 
-            if (cachedDiffData && !cachedDiffData.isBinary && !isImageFile(filePath)) {
+            if (cachedDiffData?.patch && !cachedDiffData.isBinary && !isImageFile(filePath)) {
+                targetLine = getFirstChangedModifiedLineFromPatch(cachedDiffData.patch);
+            } else if (cachedDiffData && cachedDiffData.contextMode === 'full' && !cachedDiffData.isBinary && !isImageFile(filePath)) {
                 targetLine = getFirstChangedModifiedLine(cachedDiffData.original, cachedDiffData.modified);
             }
 
@@ -1433,7 +1418,7 @@ export const DiffView: React.FC<DiffViewProps> = ({
                         staged: activeDiffStaged,
                         contextLines: 3,
                     });
-                    targetLine = getFirstVisibleModifiedLineFromPatch(patchResponse.diff);
+                    targetLine = getFirstChangedModifiedLineFromPatch(patchResponse.diff);
                 } catch {
                     targetLine = null;
                 }

packages/ui/src/components/views/diffPatchUtils.ts

@@ -0,0 +1,36 @@
+export const getFirstChangedModifiedLineFromPatch = (patch: string): number | null => {
+  if (!patch) {
+    return null;
+  }
+
+  const lines = patch.split('\n');
+  let modifiedLine: number | null = null;
+
+  for (const line of lines) {
+    const hunkMatch = line.match(/^@@\s*-\d+(?:,\d+)?\s+\+(\d+)(?:,\d+)?\s*@@/);
+    if (hunkMatch) {
+      const parsed = Number.parseInt(hunkMatch[1] ?? '', 10);
+      modifiedLine = Number.isFinite(parsed) && parsed >= 1 ? parsed : null;
+      continue;
+    }
+
+    if (modifiedLine === null) {
+      continue;
+    }
+
+    if (line.startsWith(' ')) {
+      modifiedLine += 1;
+      continue;
+    }
+
+    if (line.startsWith('+')) {
+      return modifiedLine;
+    }
+
+    if (line.startsWith('-')) {
+      return Math.max(1, modifiedLine);
+    }
+  }
+
+  return null;
+};