feat(diff): add line-level composer actions in diff viewer

Author: Dimillian

src/features/design-system/diff/diffViewerTheme.ts

@@ -10,6 +10,12 @@ export const DIFF_VIEWER_SCROLL_CSS = `
   background-image: none !important;
 }
 
+[data-hover-slot] {
+  left: 0 !important;
+  right: auto !important;
+  justify-content: flex-start !important;
+}
+
 diffs-container,
 [data-diffs],
 [data-diffs-header],

src/features/git/components/DiffBlock.tsx

@@ -1,7 +1,7 @@
 /** @vitest-environment jsdom */
-import { render, screen } from "@testing-library/react";
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
 import type { ReactNode } from "react";
-import { beforeAll, describe, expect, it, vi } from "vitest";
+import { afterEach, beforeAll, describe, expect, it, vi } from "vitest";
 import { GitDiffViewer } from "./GitDiffViewer";
 
 vi.mock("@tanstack/react-virtual", () => ({
@@ -18,11 +18,41 @@ vi.mock("@tanstack/react-virtual", () => ({
 }));
 
 vi.mock("@pierre/diffs", () => ({
-  parsePatchFiles: () => [],
+  parsePatchFiles: (diff: string) =>
+    diff.includes("@@")
+      ? [
+          {
+            files: [
+              {
+                name: "src/main.ts",
+                prevName: undefined,
+                type: "change",
+                hunks: [],
+                splitLineCount: 0,
+                unifiedLineCount: 0,
+              },
+            ],
+          },
+        ]
+      : [],
 }));
 
 vi.mock("@pierre/diffs/react", () => ({
-  FileDiff: () => null,
+  FileDiff: ({
+    renderHoverUtility,
+  }: {
+    renderHoverUtility?: (
+      getHoveredLine: () =>
+        | { lineNumber: number; side?: "additions" | "deletions" }
+        | undefined,
+    ) => ReactNode;
+  }) => (
+    <div>
+      {renderHoverUtility
+        ? renderHoverUtility(() => ({ lineNumber: 2, side: "additions" }))
+        : null}
+    </div>
+  ),
   WorkerPoolContextProvider: ({ children }: { children: ReactNode }) => children,
 }));
 
@@ -38,7 +68,42 @@ beforeAll(() => {
     ResizeObserverMock;
 });
 
+afterEach(() => {
+  cleanup();
+});
+
 describe("GitDiffViewer", () => {
+  it("inserts a diff line reference into composer when the line '+' action is clicked", () => {
+    const onInsertComposerText = vi.fn();
+
+    render(
+      <GitDiffViewer
+        diffs={[
+          {
+            path: "src/main.ts@@item-change-1@@change-0",
+            displayPath: "src/main.ts",
+            status: "M",
+            diff: "@@ -1,1 +1,2 @@\n line one\n+added line",
+          },
+        ]}
+        selectedPath="src/main.ts@@item-change-1@@change-0"
+        isLoading={false}
+        error={null}
+        diffStyle="unified"
+        onInsertComposerText={onInsertComposerText}
+      />,
+    );
+
+    fireEvent.click(
+      screen.getByRole("button", { name: "Ask for changes on hovered line" }),
+    );
+
+    expect(onInsertComposerText).toHaveBeenCalledTimes(1);
+    expect(onInsertComposerText).toHaveBeenCalledWith(
+      "src/main.ts:L2\n```diff\n+added line\n```\n\n",
+    );
+  });
+
   it("renders raw fallback lines instead of Diff unavailable for non-patch diffs", () => {
     render(
       <GitDiffViewer