feat(project-tree): Exchange files with the OS clipboard on Windows (#41)

Exchange files with the OS clipboard on Windows: CF_HDROP read/write in a new ide-clipboard crate (Win32 FFI behind cfg(windows), stub elsewhere), three thin commands, and project-view copy/paste hooks. Pasting from Explorer always copies; our own hand-off falls back to the internal clipboard so Copy/Move dialogs and refactoring stay intact.
Verified: ide-clipboard round-trip tests, project-tree vitest (93), CI on all three OSes. Not verified on Windows hardware here.

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/features/project-tree/ProjectView.test.tsx

@@ -4,6 +4,7 @@ import { emitLocalIdeEvent } from "@/platform/ipc";
 import { getContext } from "@/platform/context";
 import { provideEditorService, type EditorService } from "@/workbench/contributions";
 import { activate } from "./index";
+import { setOsClipboardForTests } from "./clipboard";
 import { MemoryFs, setProjectFsForTests } from "./fs";
 import { MessagesHost } from "@/workbench/messages";
 import { FileRefactorDialogHost } from "./FileRefactorDialogs";
@@ -71,6 +72,29 @@ describe("ProjectView", () => {
     await waitFor(() => expect(rows()).toEqual(["acme/w/acme", "build", "src", "App.kt", "Util.kt", "README.md"]));
   });
 
+  it("pastes the files Explorer copied into the selected folder, as copies", async () => {
+    // What Explorer leaves on the clipboard: a path outside the project.
+    fs.nodes.set("/elsewhere/Photo.png", { kind: "file", size: 3 });
+    setOsClipboardForTests({
+      files: async () => ["/elsewhere/Photo.png"],
+      hasFiles: async () => true,
+      setFiles: async () => {},
+    });
+    try {
+      const { tree, row, rows } = setup();
+      await waitFor(() => expect(rows()).toContain("README.md"));
+      fireEvent.mouseDown(row("src"));
+      tree.focus();
+      // ⌘ is the primary modifier here, as in the undo test below.
+      fireEvent.keyDown(tree, { key: "v", code: "KeyV", metaKey: true });
+      await waitFor(() => expect(fs.nodes.has("/w/acme/src/Photo.png")).toBe(true));
+      // A copy, not a move: the dropped-from path stays where it was.
+      expect(fs.nodes.has("/elsewhere/Photo.png")).toBe(true);
+    } finally {
+      setOsClipboardForTests(null);
+    }
+  });
+
   it("opens files on double click and previews on single click only when enabled", async () => {
     const { row, rows } = setup();
     await waitFor(() => expect(rows()).toContain("README.md"));

src/features/project-tree/ProjectView.tsx

@@ -59,6 +59,7 @@ import {
   openRenameDialog,
   openSelection,
   paste,
+  refreshOsClipboard,
   targetFolder,
 } from "./operations";
 import { isInside, nameOf, parentOf, splitExtension, tildify } from "./paths";
@@ -504,6 +505,15 @@ export function ProjectView() {
     };
   }, []);
 
+  // Files copied in Explorer keep Paste usable: the OS clipboard can change
+  // while this window is in the background, so re-ask whenever it comes back.
+  useEffect(() => {
+    void refreshOsClipboard();
+    const refresh = () => void refreshOsClipboard();
+    window.addEventListener("focus", refresh);
+    return () => window.removeEventListener("focus", refresh);
+  }, []);
+
   const onKeyDown = (e: ReactKeyboardEvent<HTMLDivElement>) => {
     if ((e.target as HTMLElement).tagName === "INPUT") return;
     const stroke = strokeOf(e.nativeEvent);

src/features/project-tree/clipboard.ts

@@ -0,0 +1,57 @@
+/**
+ * The OS clipboard's file list: what Explorer copies, and what the project view
+ * puts there so Explorer can paste it back. The format is native (Windows
+ * `CF_HDROP`), so the backend owns it; a plain browser has nothing to exchange
+ * and every call degrades to "no files".
+ */
+import { call, commands, isTauri } from "@/platform/ipc";
+
+export interface OsClipboard {
+  /** Files Explorer copied; empty when the clipboard holds none. */
+  files(): Promise<string[]>;
+  /** Whether the clipboard holds files: cheap, for the Paste item's enabled state. */
+  hasFiles(): Promise<boolean>;
+  /** Puts `paths` there, so Explorer can paste them. */
+  setFiles(paths: string[]): Promise<void>;
+}
+
+const native: OsClipboard = {
+  async files() {
+    if (!isTauri()) return [];
+    try {
+      return await call(commands.clipboardFileList());
+    } catch {
+      // A clipboard another app holds for a moment is not an error worth showing.
+      return [];
+    }
+  },
+
+  async hasFiles() {
+    if (!isTauri()) return false;
+    try {
+      return await commands.clipboardHasFiles();
+    } catch {
+      return false;
+    }
+  },
+
+  async setFiles(paths) {
+    if (!isTauri() || !paths.length) return;
+    try {
+      await call(commands.clipboardSetFileList(paths));
+    } catch {
+      // The internal clipboard still works; a failed hand-off is not an error.
+    }
+  },
+};
+
+let current = native;
+
+export function osClipboard(): OsClipboard {
+  return current;
+}
+
+/** Tests: force a clipboard implementation. */
+export function setOsClipboardForTests(clipboard: OsClipboard | null) {
+  current = clipboard ?? native;
+}

src/features/project-tree/operations.ts

@@ -7,6 +7,7 @@ import { askUsagesDetected, fileUsages, viewUsages } from "@/editor/refactor";
 import { executeAction, getAction } from "@/platform/actions";
 import { maybeEditorService, notifications } from "@/workbench/contributions";
 import { ask, confirmChecked } from "@/workbench/messages";
+import { osClipboard } from "./clipboard";
 import { runFileRefactoring } from "./fileRefactor";
 import { openFileRefactorDialog, rememberedOptions } from "./FileRefactorDialogs";
 import { projectFs } from "./fs";
@@ -455,17 +456,36 @@ export async function deleteSelection(nodes: ProjectNode[] = selectedNodes()) {
 
 export function copySelection(cut: boolean) {
   const paths = topLevelPaths(selectedNodes().filter((n) => (cut ? mutable(n) : !n.root && !n.group)));
-  if (paths.length) setClipboard({ paths, cut });
+  if (!paths.length) return;
+  setClipboard({ paths, cut });
+  // Hand the files to the OS too, so Explorer can paste them (as IDEA does).
+  void osClipboard().setFiles(paths);
+}
+
+/** Files on the OS clipboard, cached: `canPaste` runs while a menu renders. */
+let osHasFiles = false;
+
+/** Keeps Paste enabled for what Explorer copied; called when the window gains focus. */
+export async function refreshOsClipboard() {
+  osHasFiles = await osClipboard().hasFiles();
 }
 
 export function canPaste(): boolean {
-  return !!get().clipboard?.paths.length;
+  return !!get().clipboard?.paths.length || osHasFiles;
 }
 
-/** Paste: cut items go through the Move dialog, copied ones through the Copy dialog (IDEA). */
+/**
+ * Paste: cut items go through the Move dialog, copied ones through the Copy
+ * dialog (IDEA). Files copied in Explorer paste as copies.
+ */
 export async function paste(target = targetFolder()) {
   const { clipboard } = get();
-  if (!clipboard || !target || isReadOnlyPath(target)) return;
+  if (!target || isReadOnlyPath(target)) return;
+  const external = await osClipboard().files();
+  // The last copy is the one the user expects to paste, so another app's files
+  // win over ours — unless they are the copy we handed to the OS ourselves.
+  if (external.length && !sameFiles(external, clipboard?.paths)) return pasteFromOsClipboard(external, target);
+  if (!clipboard?.paths.length) return;
   const isDir = (p: string) => get().listings.has(p) || currentModel().byKey.get(currentModel().keyOfPath.get(p) ?? p)?.dir === true;
   const single = clipboard.paths.length === 1 && isDir(clipboard.paths[0]!);
   if (!clipboard.cut) return openCopyDialogFor(clipboard.paths, target, single);
@@ -476,6 +496,19 @@ export async function paste(target = targetFolder()) {
   });
 }
 
+/** Pasting from Explorer copies, whatever the source: never a move or a refactoring. */
+async function pasteFromOsClipboard(paths: string[], target: string) {
+  osHasFiles = true;
+  await transfer(paths, target, "copy");
+}
+
+/** The same files, as the clipboard keeps them (`/`-separated, any order). */
+function sameFiles(files: string[], paths: string[] | undefined): boolean {
+  if (!paths || files.length !== paths.length) return false;
+  const key = (list: string[]) => list.map((p) => (caseInsensitive ? p.toLowerCase() : p)).sort().join("\n");
+  return key(files) === key(paths);
+}
+
 function openCopyDialogFor(paths: string[], target: string, dir?: boolean) {
   if (!paths.length) return;
   void loadDir(target);