chat: fix undo/redo skipping multiple no-edit requests (#299330)

- Fixes _willRedoToEpoch to advance one request at a time when there are
  no edit operations ahead, instead of jumping past all remaining requests
- When redoing with no operations in the queue, now finds the next
  request-start checkpoint boundary and advances there, following the same
  single-step pattern as undo
- Adds test case verifying undo and redo step through consecutive no-edit
  requests one at a time

Fixes #275234

(Commit message generated by Copilot)

src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingCheckpointTimelineImpl.ts

@@ -123,7 +123,19 @@ export class ChatEditingCheckpointTimelineImpl implements IChatEditingCheckpoint
 
 		// Find the next edit operation that would be applied...
 		const nextOperation = operations.find(op => op.epoch >= currentEpoch);
-		const nextCheckpoint = nextOperation && checkpoints.find(op => op.epoch > nextOperation.epoch);
+
+		// When there are no more operations, advance one request at a time
+		// by finding the next request-start checkpoint boundary.
+		if (!nextOperation) {
+			const nextRequestStart = checkpoints.find(cp => cp.epoch >= currentEpoch && cp.undoStopId === undefined);
+			if (!nextRequestStart) {
+				return maxEncounteredEpoch + 1;
+			}
+			const requestAfter = checkpoints.find(cp => cp.epoch > nextRequestStart.epoch && cp.undoStopId === undefined);
+			return requestAfter ? requestAfter.epoch : (maxEncounteredEpoch + 1);
+		}
+
+		const nextCheckpoint = checkpoints.find(op => op.epoch > nextOperation.epoch);
 
 		// And figure out where we're going if we're navigating across request
 		// 1. If there is no next request or if the next target checkpoint is in

src/vs/workbench/contrib/chat/test/browser/chatEditing/chatEditingCheckpointTimeline.test.ts

@@ -1248,6 +1248,54 @@ suite('ChatEditingCheckpointTimeline', function () {
 		await timeline.navigateToCheckpoint(stop2NewCheckpointId);
 		assert.strictEqual(fileContents.get(uri), 'replacement edit', 'Content should still be correct after full timeline traversal');
 	});
+
+	test('undo/redo with multiple no-edit requests advances one request at a time', async function () {
+		// req1: no edits
+		timeline.createCheckpoint('req1', undefined, 'Start req1');
+
+		// req2: no edits
+		timeline.createCheckpoint('req2', undefined, 'Start req2');
+
+		// req3: no edits
+		timeline.createCheckpoint('req3', undefined, 'Start req3');
+
+		// req4: no edits
+		timeline.createCheckpoint('req4', undefined, 'Start req4');
+
+		// Undo should step one request at a time
+		assert.strictEqual(timeline.canUndo.get(), true);
+
+		await timeline.undoToLastCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), ['req4']);
+
+		await timeline.undoToLastCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), ['req4', 'req3']);
+
+		await timeline.undoToLastCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), ['req4', 'req3', 'req2']);
+
+		await timeline.undoToLastCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), ['req4', 'req3', 'req2', 'req1']);
+
+		assert.strictEqual(timeline.canUndo.get(), false);
+
+		// Redo should also step one request at a time (not skip all at once)
+		assert.strictEqual(timeline.canRedo.get(), true);
+
+		await timeline.redoToNextCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), ['req4', 'req3', 'req2']);
+
+		await timeline.redoToNextCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), ['req4', 'req3']);
+
+		await timeline.redoToNextCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), ['req4']);
+
+		await timeline.redoToNextCheckpoint();
+		assert.deepStrictEqual(timeline.requestDisablement.get().map(d => d.requestId), []);
+
+		assert.strictEqual(timeline.canRedo.get(), false);
+	});
 });
 
 // Mock notebook service for tests that don't need notebook functionality