Fix two bugs in microsoft#214589.

1. We must not `dispose()` the `MutableDisposable` `this._commentThreadWidget` - as that disposes the MutableDisposable itself, and it cannot then later be reassigned to a new value. Instead, we need to assign `value=undefined`, which disposes the previous `value`, but keeps the `MutableDisposable` available to be reused later.
2. `initialize()` is a no-op if `this.currentElement` is already identical to the passed `element`, so we must not do that assignment before calling initialize - instead `initialize()` does the assignment after checking.

src/vs/workbench/contrib/notebook/browser/view/cellParts/cellComments.ts

@@ -108,7 +108,7 @@ export class CellComments extends CellContentPart {
 		if (this._commentThreadWidget.value) {
 			if (!info) {
 				this._commentThreadDisposables.clear();
-				this._commentThreadWidget.dispose();
+				this._commentThreadWidget.value = undefined;
 				this.currentElement.commentHeight = 0;
 				return;
 			}
@@ -154,7 +154,6 @@ export class CellComments extends CellContentPart {
 
 	override didRenderCell(element: ICellViewModel): void {
 		if (element.cellKind === CellKind.Code) {
-			this.currentElement = element as CodeCellViewModel;
 			this.initialize(element);
 			this._bindListeners();
 		}