Implement M2.1 and M2.2: multiple-selection engine and UX

M2.1 — Multiple-Selection Engine:
- Enable native multi-cursor/column selection via allowMultipleSelections,
  drawSelection, and @codemirror/search in the base extension group
- Add @codemirror/search as a direct dependency
- Add editorSelectionOps.ts: select-next, select-all, skip, and
  remove-last occurrence operations with deterministic wrap behavior
- Wire occurrence actions into domain APIs, runner adapter, and capability
- Verify line operations (move/duplicate/join) are multi-range safe with
  extended test coverage; document search-nav selection replacement

M2.2 — Multiple-Selection Commands and UX:
- Add 4 new commands (selectNextOccurrence, selectAllOccurrences,
  skipOccurrence, undoOccurrence) with definitions, handlers, menu items
- Reassign Cmd/Ctrl+D from duplicateLine to selectNextOccurrence;
  duplicate line moves to Cmd/Ctrl+Alt+D (Shift+D owned by diff preview)
- Extend cursor status, app state, and status bar to show selection count
  (compact 'N selections' segment, aria-labeled, active-pane only)
- Add cross-feature validation tests for multi-cursor undo, synchronized
  typing, occurrence non-overlap, and save-content parity

All 2750 tests pass; npm run check reports 0 new errors.

Author: Alexey Perov

app/package-lock.json

@@ -30,6 +30,7 @@
     "@codemirror/lang-yaml": "^6.1.3",
     "@codemirror/language": "^6.12.3",
     "@codemirror/legacy-modes": "^6.5.3",
+    "@codemirror/search": "^6.5.6",
     "@codemirror/state": "^6.6.0",
     "@codemirror/view": "^6.43.0",
     "@lezer/highlight": "^1.2.3",

app/package.json

@@ -328,7 +328,7 @@ export const commandDefinitions: CommandDefinition[] = [
     category: "Edit",
     paletteIntent: "palette",
     availability: "document",
-    binding: { mac: "Cmd+D", windows: "Ctrl+D" },
+    binding: { mac: "Cmd+Alt+D", windows: "Ctrl+Alt+D" },
   },
   {
     id: "edit.joinLines",
@@ -339,6 +339,46 @@ export const commandDefinitions: CommandDefinition[] = [
     availability: "document",
     binding: { mac: "Cmd+J", windows: "Ctrl+J" },
   },
+  {
+    id: "edit.selectNextOccurrence",
+    label: "Select Next Occurrence",
+    menuPath: "Edit/Select Next Occurrence",
+    category: "Edit",
+    searchTerms: ["find", "match", "multi-cursor"],
+    paletteIntent: "palette",
+    availability: "document",
+    binding: { mac: "Cmd+D", windows: "Ctrl+D" },
+  },
+  {
+    id: "edit.selectAllOccurrences",
+    label: "Select All Occurrences",
+    menuPath: "Edit/Select All Occurrences",
+    category: "Edit",
+    searchTerms: ["find", "match", "multi-cursor"],
+    paletteIntent: "palette",
+    availability: "document",
+    binding: { mac: "Cmd+Shift+L", windows: "Ctrl+Shift+L" },
+  },
+  {
+    id: "edit.skipOccurrence",
+    label: "Skip Occurrence",
+    menuPath: "Edit/Skip Occurrence",
+    category: "Edit",
+    searchTerms: ["find", "match"],
+    paletteIntent: "palette",
+    availability: "document",
+    binding: { mac: "none", windows: "none" },
+  },
+  {
+    id: "edit.undoOccurrence",
+    label: "Remove Last Occurrence",
+    menuPath: "Edit/Remove Last Occurrence",
+    category: "Edit",
+    searchTerms: ["find", "match", "remove", "cursor"],
+    paletteIntent: "palette",
+    availability: "document",
+    binding: { mac: "none", windows: "none" },
+  },
   {
     id: "view.toggleWrap",
     label: "Toggle Wrap",

app/src/lib/commands/definitions.ts

@@ -176,6 +176,10 @@ function createEditorRunnerMock(): EditorCommandRunner {
     moveLineDown: vi.fn(),
     duplicateLine: vi.fn(),
     joinLines: vi.fn(),
+    selectNextOccurrence: vi.fn(() => false),
+    selectAllOccurrences: vi.fn(() => false),
+    skipOccurrence: vi.fn(() => false),
+    undoOccurrence: vi.fn(() => false),
     setWrap: vi.fn(),
     setZoom: vi.fn(),
     findNext: vi.fn(() => false),
@@ -549,6 +553,10 @@ describe("edit commands", () => {
     dispatchCommand("edit.moveLineDown", context);
     dispatchCommand("edit.duplicateLine", context);
     dispatchCommand("edit.joinLines", context);
+    dispatchCommand("edit.selectNextOccurrence", context);
+    dispatchCommand("edit.selectAllOccurrences", context);
+    dispatchCommand("edit.skipOccurrence", context);
+    dispatchCommand("edit.undoOccurrence", context);
 
     expect(editorRunner.undo).toHaveBeenCalled();
     expect(editorRunner.redo).toHaveBeenCalled();
@@ -558,6 +566,10 @@ describe("edit commands", () => {
     expect(editorRunner.moveLineDown).toHaveBeenCalled();
     expect(editorRunner.duplicateLine).toHaveBeenCalled();
     expect(editorRunner.joinLines).toHaveBeenCalled();
+    expect(editorRunner.selectNextOccurrence).toHaveBeenCalled();
+    expect(editorRunner.selectAllOccurrences).toHaveBeenCalled();
+    expect(editorRunner.skipOccurrence).toHaveBeenCalled();
+    expect(editorRunner.undoOccurrence).toHaveBeenCalled();
   });
 
   it("no-ops when no editor runner is available", () => {
@@ -746,6 +758,10 @@ describe("command dispatch coverage", () => {
       "edit.moveLineDown",
       "edit.duplicateLine",
       "edit.joinLines",
+      "edit.selectNextOccurrence",
+      "edit.selectAllOccurrences",
+      "edit.skipOccurrence",
+      "edit.undoOccurrence",
       "view.toggleWrap",
       "view.zoomIn",
       "view.zoomOut",

app/src/lib/commands/handlers/appViewHandlers.test.ts

@@ -25,4 +25,16 @@ export const editHandlers: CommandHandlerMap = {
   "edit.joinLines": ({ getEditorRunner }) => {
     getEditorRunner()?.joinLines();
   },
+  "edit.selectNextOccurrence": ({ getEditorRunner }) => {
+    getEditorRunner()?.selectNextOccurrence();
+  },
+  "edit.selectAllOccurrences": ({ getEditorRunner }) => {
+    getEditorRunner()?.selectAllOccurrences();
+  },
+  "edit.skipOccurrence": ({ getEditorRunner }) => {
+    getEditorRunner()?.skipOccurrence();
+  },
+  "edit.undoOccurrence": ({ getEditorRunner }) => {
+    getEditorRunner()?.undoOccurrence();
+  },
 };

app/src/lib/commands/handlers/edit.ts

@@ -176,6 +176,10 @@ function createEditorRunnerMock(): EditorCommandRunner {
     moveLineDown: vi.fn(),
     duplicateLine: vi.fn(),
     joinLines: vi.fn(),
+    selectNextOccurrence: vi.fn(() => false),
+    selectAllOccurrences: vi.fn(() => false),
+    skipOccurrence: vi.fn(() => false),
+    undoOccurrence: vi.fn(() => false),
     setWrap: vi.fn(),
     setZoom: vi.fn(),
     findNext: vi.fn(() => false),

app/src/lib/commands/handlers/fileHandlers.test.ts

@@ -175,6 +175,10 @@ function createEditorRunnerMock(): EditorCommandRunner {
     moveLineDown: vi.fn(),
     duplicateLine: vi.fn(),
     joinLines: vi.fn(),
+    selectNextOccurrence: vi.fn(() => false),
+    selectAllOccurrences: vi.fn(() => false),
+    skipOccurrence: vi.fn(() => false),
+    undoOccurrence: vi.fn(() => false),
     setWrap: vi.fn(),
     setZoom: vi.fn(),
     findNext: vi.fn(() => false),

app/src/lib/commands/handlers/keymapHandlers.test.ts

@@ -175,6 +175,10 @@ function createEditorRunnerMock(): EditorCommandRunner {
     moveLineDown: vi.fn(),
     duplicateLine: vi.fn(),
     joinLines: vi.fn(),
+    selectNextOccurrence: vi.fn(() => false),
+    selectAllOccurrences: vi.fn(() => false),
+    skipOccurrence: vi.fn(() => false),
+    undoOccurrence: vi.fn(() => false),
     setWrap: vi.fn(),
     setZoom: vi.fn(),
     findNext: vi.fn(() => false),

app/src/lib/commands/handlers/workspaceHandlers.test.ts

@@ -156,6 +156,7 @@
     zoomPercent: number;
     cursorLine: number;
     cursorColumn: number;
+    selectionCount: number;
     decoratePlaintextSymbols: boolean;
     showMinimap: boolean;
     maxBinaryOpenAsTextBytes: number;
@@ -581,6 +582,14 @@
           <span class="status-segment optional-segment optional-cursor">
             Ln {editor.cursorLine}, Col {editor.cursorColumn}
           </span>
+          {#if editor.selectionCount > 1}
+            <span
+              class="status-segment optional-segment optional-selections"
+              aria-label={`${editor.selectionCount} selections`}
+            >
+              {editor.selectionCount} selections
+            </span>
+          {/if}
           <span class="status-segment optional-segment optional-encoding">
             {#if editor.isImageDocument}
               Image

app/src/lib/components/AppShell.svelte

@@ -34,6 +34,10 @@ export type AppCommandId =
   | "edit.moveLineDown"
   | "edit.duplicateLine"
   | "edit.joinLines"
+  | "edit.selectNextOccurrence"
+  | "edit.selectAllOccurrences"
+  | "edit.skipOccurrence"
+  | "edit.undoOccurrence"
   | "view.toggleWrap"
   | "view.zoomIn"
   | "view.zoomOut"