fix reading tabs config

src/vs/workbench/browser/parts/editor/editorPart.ts

@@ -128,6 +128,8 @@ export class EditorPart extends Part implements IEditorPart, IEditorGroupService
 
 		this.stacks = this.instantiationService.createInstance(EditorStacksModel);
 
+		this.previewEditors = configurationService.getConfiguration<IWorkbenchEditorConfiguration>().workbench.previewEditors;
+
 		this.registerListeners();
 	}
 

src/vs/workbench/common/editor/editorStacksModel.ts

@@ -88,7 +88,7 @@ export class EditorGroup implements IEditorGroup {
 		this.mru = [];
 		this.toDispose = [];
 		this.mapResourceToEditor = Object.create(null);
-		this.editorOpenPositioning = configurationService.getConfiguration<IWorkbenchEditorConfiguration>().workbench.editorOpenPositioning;
+		this.onConfigurationUpdated(configurationService.getConfiguration<IWorkbenchEditorConfiguration>());
 
 		this._onEditorActivated = new Emitter<EditorInput>();
 		this._onEditorOpened = new Emitter<EditorInput>();