fix(desktop): surface diff type badge in card title when file header is collapsed

Instead of forcing the per-file header open just to show the New
file/Deleted badge (previous fix), keep the header collapsed when it
would only repeat the card title, and show the badge in the diff card's
title bar instead — both in the inline DiffMessage card and the expanded
dialog.

DIFF_TYPE_LABELS and the header-visibility check move into parseDiff.ts
as shared helpers, and a new getDiffTitleBadge returns the badge label
only for single-file diffs whose collapsed header has a notable change
type — so the badge never renders in both places.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Matt Toohey <contact@matttoohey.com>

desktop/src/features/messages/lib/parseDiff.ts

@@ -33,6 +33,14 @@ export function parseUnifiedDiff(content: string): ParsedDiffResult {
   }
 }
 
+export const DIFF_TYPE_LABELS: Record<DiffType, string> = {
+  add: "New file",
+  copy: "Copied",
+  delete: "Deleted",
+  modify: "Modified",
+  rename: "Renamed",
+};
+
 export function getDiffFileLabel(
   file: FileData,
   fallbackFilePath?: string,
@@ -47,6 +55,39 @@ export function getDiffFileLabel(
   return newPath || oldPath || fallbackFilePath || "diff";
 }
 
+export function shouldShowDiffFileHeader(
+  label: string,
+  fileCount: number,
+  fallbackFilePath?: string,
+): boolean {
+  return fileCount > 1 || !fallbackFilePath || label !== fallbackFilePath;
+}
+
+/**
+ * Badge for the diff card's title bar. Set only when the diff is a single
+ * file whose per-file header is collapsed (its label just repeats the card
+ * title) and whose change type is notable — so "New file"/"Deleted" isn't
+ * lost with the header.
+ */
+export function getDiffTitleBadge(
+  content: string,
+  fallbackFilePath?: string,
+): string | undefined {
+  const { files } = parseUnifiedDiff(content);
+  if (files.length !== 1) {
+    return undefined;
+  }
+
+  const file = files[0];
+  const label = getDiffFileLabel(file, fallbackFilePath);
+  if (shouldShowDiffFileHeader(label, files.length, fallbackFilePath)) {
+    return undefined;
+  }
+
+  const diffType = normalizeDiffType(file.type);
+  return diffType === "modify" ? undefined : DIFF_TYPE_LABELS[diffType];
+}
+
 export function countDiffFileChanges(file: FileData) {
   let additions = 0;
   let deletions = 0;

desktop/src/features/messages/ui/DiffMessage.tsx

@@ -1,6 +1,7 @@
 import * as React from "react";
 import { FileDiff, Maximize2 } from "lucide-react";
 
+import { getDiffTitleBadge } from "@/features/messages/lib/parseDiff";
 import { isSafeUrl } from "@/shared/lib/url";
 import { Button } from "@/shared/ui/button";
 import { useSmoothCorners } from "@/shared/ui/smoothCorners";
@@ -39,6 +40,11 @@ export default function DiffMessage({
 
   const safeRepoUrl = isSafeUrl(repoUrl) ? repoUrl : undefined;
 
+  const titleBadge = React.useMemo(
+    () => getDiffTitleBadge(content, filePath),
+    [content, filePath],
+  );
+
   const commitUrl =
     safeRepoUrl && commitSha ? `${safeRepoUrl}/commit/${commitSha}` : undefined;
 
@@ -54,6 +60,11 @@ export default function DiffMessage({
         <span className="flex-1 truncate font-mono text-xs text-foreground/80">
           {filePath ?? "diff"}
         </span>
+        {titleBadge && (
+          <span className="shrink-0 rounded-md border border-border/60 px-1.5 py-0.5 text-2xs uppercase tracking-[0.14em] text-muted-foreground">
+            {titleBadge}
+          </span>
+        )}
         {shortSha && (
           <span className="text-xs text-muted-foreground font-mono">
             {commitUrl ? (

desktop/src/features/messages/ui/DiffMessageExpanded.tsx

@@ -1,6 +1,7 @@
 import { Rows3, SplitSquareVertical } from "lucide-react";
-import { useState } from "react";
+import { useMemo, useState } from "react";
 
+import { getDiffTitleBadge } from "@/features/messages/lib/parseDiff";
 import { DiffViewer } from "@/features/messages/ui/DiffViewer";
 import { Button } from "@/shared/ui/button";
 import {
@@ -23,6 +24,11 @@ export default function DiffMessageExpanded({
 }: DiffMessageExpandedProps) {
   const [viewType, setViewType] = useState<"split" | "unified">("unified");
 
+  const titleBadge = useMemo(
+    () => getDiffTitleBadge(content, filePath),
+    [content, filePath],
+  );
+
   return (
     <Dialog
       onOpenChange={(open) => {
@@ -36,6 +42,11 @@ export default function DiffMessageExpanded({
             <DialogTitle className="min-w-0 flex-1 truncate font-mono text-sm font-medium">
               {filePath ?? "Diff Viewer"}
             </DialogTitle>
+            {titleBadge && (
+              <span className="shrink-0 rounded-md border border-border/60 px-1.5 py-0.5 text-2xs uppercase tracking-[0.14em] text-muted-foreground">
+                {titleBadge}
+              </span>
+            )}
             <div className="flex items-center gap-1 rounded-lg border border-border/60 bg-muted/30 p-1">
               <Button
                 className="h-7 px-2"

desktop/src/features/messages/ui/DiffViewer.tsx

@@ -4,9 +4,11 @@ import { useMemo } from "react";
 
 import {
   countDiffFileChanges,
+  DIFF_TYPE_LABELS,
   getDiffFileLabel,
   normalizeDiffType,
   parseUnifiedDiff,
+  shouldShowDiffFileHeader,
 } from "@/features/messages/lib/parseDiff";
 import { cn } from "@/shared/lib/cn";
 import "./DiffViewer.css";
@@ -18,14 +20,6 @@ type DiffViewerProps = {
   className?: string;
 };
 
-const DIFF_TYPE_LABELS = {
-  add: "New file",
-  copy: "Copied",
-  delete: "Deleted",
-  modify: "Modified",
-  rename: "Renamed",
-} as const;
-
 function FileChangeBadge({
   tone,
   value,
@@ -81,11 +75,11 @@ export function DiffViewer({
           const label = getDiffFileLabel(file, fallbackFilePath);
           const { additions, deletions } = countDiffFileChanges(file);
           const diffType = normalizeDiffType(file.type);
-          const showFileHeader =
-            files.length > 1 ||
-            !fallbackFilePath ||
-            label !== fallbackFilePath ||
-            diffType !== "modify";
+          const showFileHeader = shouldShowDiffFileHeader(
+            label,
+            files.length,
+            fallbackFilePath,
+          );
           const fileKey = [
             file.oldPath || "",
             file.newPath || "",