feat(vcs): Accept a diff change from the keyboard and step between diff files (#158)

- Accept Left Side / Accept Right Side copy the change at the caret to the
  other side of a side-by-side diff (⌃⌘→ / ⌃⌘← on macOS, Ctrl+Alt+R /
  Ctrl+Alt+A on Windows).
- Compare Next File / Compare Previous File open the neighbouring file of
  the diff chain directly (⌃⇧→ / ⌃⇧← on macOS, Alt+Shift+→ / ← on Windows).

src/features/settings/__tests__/downloadJdk.test.tsx

@@ -66,6 +66,7 @@ describe("Download JDK dialog", () => {
   it("preselects the version a fix asks for", async () => {
     render(<DownloadJdkDialog initialVersion={21} onClose={() => {}} onDownloaded={() => {}} />);
     await waitFor(() => expect(screen.getByRole("button", { name: "Download" })).toBeEnabled());
-    expect(screen.getByRole("button", { name: "Version" })).toHaveTextContent("21");
+    // The version list loads after the Download button enables on a busy runner.
+    await waitFor(() => expect(screen.getByRole("button", { name: "Version" })).toHaveTextContent("21"));
   });
 });

src/features/vcs/DiffEditor.tsx

@@ -24,7 +24,7 @@ import { tx } from "@/platform/i18n";
 import { IconButton, Select, Spinner, T } from "@/ui";
 import { editorService, type EditorInput } from "@/workbench/contributions";
 import { languageFor, viewerBase } from "./cm";
-import { applyChunk, arrowControls, hoverRevert } from "./diffControls";
+import { applyChunk, arrowControls, chunkAt, hoverRevert } from "./diffControls";
 import {
   HIGHLIGHT_OPTIONS,
   IGNORE_OPTIONS,
@@ -586,10 +586,26 @@ export function DiffEditor({ input }: { input: EditorInput }) {
     e.stopPropagation();
   };
 
-  const nav = useRef<DiffNavigator>({ next: () => step(1), prev: () => step(-1), jumpToSource });
+  /** ⌃⌘→ / ⌃⌘←: the change at the caret applied toward the other, editable side. */
+  const applyChange = (source: "a" | "b") => {
+    const views = viewsRef.current;
+    const merge = views?.merge;
+    if (!merge || !views.a || !(source === "a" ? editableB : editableA)) return false;
+    const side = focusedSide.current;
+    const view = side === "a" ? views.a : views.b;
+    const index = chunkAt(merge.chunks, side, view.state.selection.main.head, view.state.doc.length);
+    if (index < 0) return false;
+    applyChunk(merge, index, source);
+    return true;
+  };
+  const file = (dir: 1 | -1) => {
+    const neighbour = chainNeighbour(data, dir);
+    if (neighbour) void openDiff(neighbour, { preview: true });
+  };
+  const nav = useRef<DiffNavigator>({ next: () => step(1), prev: () => step(-1), jumpToSource, applyChange, file });
   useEffect(() => {
-    nav.current = { next: () => step(1), prev: () => step(-1), jumpToSource };
-  }, [step, jumpToSource]);
+    nav.current = { next: () => step(1), prev: () => step(-1), jumpToSource, applyChange, file };
+  });
 
   useEffect(
     () => () => {

src/features/vcs/compare.tsx

@@ -125,6 +125,40 @@ export function registerCompareActions(): () => void {
       when: "diffEditorFocus",
       run: () => focusedDiffNavigator()?.prev(),
     },
+    {
+      id: "diff.nextFile",
+      title: "Compare Next File",
+      category: "Diff",
+      icon: "arrow-right",
+      when: "diffEditorFocus",
+      run: () => focusedDiffNavigator()?.file(1),
+    },
+    {
+      id: "diff.prevFile",
+      title: "Compare Previous File",
+      category: "Diff",
+      icon: "arrow-left",
+      when: "diffEditorFocus",
+      run: () => focusedDiffNavigator()?.file(-1),
+    },
+    {
+      id: "diff.applyLeftSide",
+      title: "Accept Left Side",
+      description: "Copies the change at the caret from the left side to the right one",
+      category: "Diff",
+      icon: "arrow-right",
+      when: "diffEditorFocus",
+      run: () => void focusedDiffNavigator()?.applyChange("a"),
+    },
+    {
+      id: "diff.applyRightSide",
+      title: "Accept Right Side",
+      description: "Copies the change at the caret from the right side to the left one",
+      category: "Diff",
+      icon: "arrow-left",
+      when: "diffEditorFocus",
+      run: () => void focusedDiffNavigator()?.applyChange("b"),
+    },
     {
       id: "diff.jumpToSource",
       title: "Jump to Source",

src/features/vcs/diffControls.ts

@@ -84,7 +84,7 @@ class RevertWidget extends WidgetType {
   }
 }
 
-function chunkAt(chunks: readonly Chunk[], side: "a" | "b", pos: number, docLength: number): number {
+export function chunkAt(chunks: readonly Chunk[], side: "a" | "b", pos: number, docLength: number): number {
   return chunks.findIndex((c) => {
     const [from, to] = side === "a" ? [c.fromA, c.toA] : [c.fromB, c.toB];
     return from === to ? pos === from : pos >= from && pos < Math.max(to, from + 1) && pos <= docLength;

src/features/vcs/diffNavigator.ts

@@ -4,6 +4,10 @@ export interface DiffNavigator {
   next(): void;
   prev(): void;
   jumpToSource(): void;
+  /** Apply the change at the caret from the left side to the right (`a`) or back (`b`). */
+  applyChange(source: "a" | "b"): boolean;
+  /** Open the next / previous file of the diff chain (changes of a commit or change list). */
+  file(dir: 1 | -1): void;
 }
 
 let focused: DiffNavigator | null = null;