Fixes #3401: [folding] Setting `editor.folding` to `false` should expand all folded regions

src/vs/editor/contrib/folding/browser/folding.ts

@@ -277,6 +277,7 @@ export class FoldingController implements editorCommon.IEditorContribution {
 				this.decorations.forEach(dec => dec.dispose(changeAccessor));
 				this.decorations = [];
 			});
+			this.updateHiddenAreas();
 		}});
 		this.localToDispose.push(this.editor.addListener2(editorCommon.EventType.MouseDown, (e) => this._onEditorMouseDown(e)));