fix: show complete diff previews

tests/regression/diff-preview-block-grouping-regression.test.mjs

@@ -30,6 +30,37 @@ const messageHandlerSource = readSource(
   "messageHandler.ts",
 );
 
+const diffPreviewStepSource = readSource(
+  [joinFromRoot("webview", "shared", "src", "chat", "components", "activity-steps", "DiffPreviewStep.tsx")],
+  "DiffPreviewStep.tsx",
+);
+
+const activityDiffExcerptSource = readSource(
+  [joinFromRoot("webview", "shared", "src", "chat", "components", "ActivityDiffExcerpt.tsx")],
+  "ActivityDiffExcerpt.tsx",
+);
+
+test("diff preview keeps the complete patch instead of applying a fixed line cap", () => {
+  assert.match(
+    diffPreviewStepSource,
+    /const excerptLines = diffLines\.filter\(\(line, index\) => index !== firstHunkIndex\)/,
+    "the parser should retain context and all later hunks",
+  );
+  assert.doesNotMatch(
+    diffPreviewStepSource,
+    /slice\(0,\s*40\)/,
+    "the parser must not truncate patches at 40 lines",
+  );
+});
+
+test("diff excerpt resets line numbering for every hunk header", () => {
+  assert.match(
+    activityDiffExcerptSource,
+    /if \(line\.startsWith\("@@"\)\)[\s\S]*?parseHunkHeader\(line\)[\s\S]*?oldN = oldStart[\s\S]*?newN = newStart/s,
+    "each hunk header should reset old and new line counters",
+  );
+});
+
 // ── parseCentralizedSessionDiffEvent ─────────────────────────────────
 
 test("parseCentralizedSessionDiffEvent handles both session.diff and message.updated event types", () => {

webview/shared/src/chat/components/ActivityDiffExcerpt.tsx

@@ -23,11 +23,25 @@ function computeLineNumbers(
   if (parsedHeader.length > 0) {
     result.push({ old: null, new: null, line: parsedHeader, isHeader: true });
   }
-  const { oldStart, newStart } =
+  let { oldStart, newStart } =
     parsedHeader.length > 0 ? parseHunkHeader(parsedHeader) : { oldStart: 1, newStart: 1 };
   let oldN = oldStart;
   let newN = newStart;
-  for (const line of lines) {
+  for (const [index, line] of lines.entries()) {
+    if (line.startsWith("@@")) {
+      // A complete patch can contain multiple hunks. Reset numbering at each
+      // subsequent header instead of treating it as a source-code line.
+      ({ oldStart, newStart } = parseHunkHeader(line));
+      oldN = oldStart;
+      newN = newStart;
+      result.push({ old: null, new: null, line, isHeader: true });
+      continue;
+    }
+    // Older callers may include the first header in `lines` as well as in
+    // `header`; avoid rendering that header twice.
+    if (index === 0 && parsedHeader.length > 0 && line === parsedHeader) {
+      continue;
+    }
     if (line.startsWith("+") && !line.startsWith("+++")) {
       result.push({ old: null, new: newN++, line, isHeader: false });
     } else if (line.startsWith("-") && !line.startsWith("---")) {

webview/shared/src/chat/components/activity-steps/DiffPreviewStep.tsx

@@ -148,8 +148,6 @@ function parsePatchTextToExcerpt(patchText?: string): DiffExcerpt | undefined {
     return undefined;
   }
 
-  const header = lines.find((line) => line.startsWith("@@")) || undefined;
-
   const diffLines = lines.filter(
     (line) =>
       !line.startsWith("*** Begin Patch") &&
@@ -163,13 +161,19 @@ function parsePatchTextToExcerpt(patchText?: string): DiffExcerpt | undefined {
       !line.startsWith("+++ "),
   );
 
-  const previewLines = diffLines.slice(0, 40);
+  const firstHunkIndex = diffLines.findIndex((line) => line.startsWith("@@"));
+  const header = firstHunkIndex >= 0 ? diffLines[firstHunkIndex] : undefined;
+
+  // Keep the complete unified diff, including context and every hunk. The
+  // modal provides scrolling, so truncating here would hide later changes.
+  // The first hunk header is supplied separately through `header`.
+  const excerptLines = diffLines.filter((line, index) => index !== firstHunkIndex);
   const added = diffLines.filter((line) => line.startsWith("+") && !line.startsWith("+++")).length;
   const deleted = diffLines.filter((line) => line.startsWith("-") && !line.startsWith("---")).length;
 
   return {
     header,
-    lines: previewLines,
+    lines: excerptLines,
     added,
     deleted,
   };