Update lengths before delta event in rollback (#22818)

During rollback of delete from string, the delta callback is invoked
before the lengths are updated, which results in the callback code
seeing messed up string content. This fix swaps the order and adds
testing for the events.

packages/dds/merge-tree/src/mergeTree.ts

@@ -2296,16 +2296,6 @@ export class MergeTree {
 				segment.removedSeq = undefined;
 				segment.localRemovedSeq = undefined;
 
-				// Note: optional chaining short-circuits:
-				// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining#short-circuiting
-				this.mergeTreeDeltaCallback?.(
-					{ op: createInsertSegmentOp(this.findRollbackPosition(segment), segment) },
-					{
-						operation: MergeTreeDeltaType.INSERT,
-						deltaSegments: [{ segment }],
-					},
-				);
-
 				for (
 					let updateNode = segment.parent;
 					updateNode !== undefined;
@@ -2317,6 +2307,16 @@ export class MergeTree {
 						this.collabWindow.clientId,
 					);
 				}
+
+				// Note: optional chaining short-circuits:
+				// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining#short-circuiting
+				this.mergeTreeDeltaCallback?.(
+					{ op: createInsertSegmentOp(this.findRollbackPosition(segment), segment) },
+					{
+						operation: MergeTreeDeltaType.INSERT,
+						deltaSegments: [{ segment }],
+					},
+				);
 			});
 		} else if (
 			op.type === MergeTreeDeltaType.INSERT ||

packages/dds/merge-tree/src/test/client.rollback.spec.ts

@@ -330,17 +330,56 @@ describe("client.rollback", () => {
 	it("Should rollback delete which causes split segments", () => {
 		client.insertTextLocal(0, "abcde");
 		client.removeRangeLocal(1, 4);
+		let deltaEvent = false;
+		client.on("delta", () => {
+			assert.equal(client.getText(), "abcde");
+			deltaEvent = true;
+		});
 		client.rollback?.({ type: MergeTreeDeltaType.REMOVE }, client.peekPendingSegmentGroups());
 
 		assert.equal(client.getText(), "abcde");
+		assert.equal(deltaEvent, true);
 	});
 	it("Should rollback delete across split segments", () => {
 		client.insertTextLocal(0, "abcde");
 		client.annotateRangeLocal(2, 3, { foo: "bar" });
 		client.removeRangeLocal(1, 4);
+
+		let deltaCount = 0;
+		let bFound = false;
+		let cFound = false;
+		let dFound = false;
+		client.on("delta", (_opArgs, delta) => {
+			deltaCount++;
+			assert.equal(client.getText().length, 2 + deltaCount);
+			assert.equal(delta.deltaSegments.length, 1);
+			assert.equal(delta.deltaSegments[0].segment.type, TextSegment.type);
+			const text = (delta.deltaSegments[0].segment as TextSegment).toString();
+			switch (text) {
+				case "b": {
+					bFound = true;
+					break;
+				}
+				case "c": {
+					cFound = true;
+					break;
+				}
+				case "d": {
+					dFound = true;
+					break;
+				}
+				default: {
+					assert(false, `Unexpected text segment: ${text}`);
+				}
+			}
+		});
 		client.rollback?.({ type: MergeTreeDeltaType.REMOVE }, client.peekPendingSegmentGroups());
 
 		assert.equal(client.getText(), "abcde");
+		assert.equal(deltaCount, 3);
+		assert(bFound);
+		assert(cFound);
+		assert(dFound);
 	});
 	it("Should rollback delete and update blocks", () => {
 		const text = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!@#$%^&*()";