panelViewlet: only update title area if no longer a singleView or becoming a singleView

fixes #45033

src/vs/workbench/browser/parts/views/panelViewlet.ts

@@ -266,10 +266,14 @@ export class PanelViewlet extends Viewlet {
 		const disposable = combinedDisposable([onDidFocus, styler, onDidChange]);
 		const panelItem: IViewletPanelItem = { panel, disposable };
 
+		const wasSingleView = this.isSingleView();
 		this.panelItems.splice(index, 0, panelItem);
 		this.panelview.addPanel(panel, size, index);
 
 		this.updateViewHeaders();
+		if (this.isSingleView() !== wasSingleView) {
+			this.updateTitleArea();
+		}
 	}
 
 	removePanel(panel: ViewletPanel): void {
@@ -283,11 +287,15 @@ export class PanelViewlet extends Viewlet {
 			this.lastFocusedPanel = undefined;
 		}
 
+		const wasSingleView = this.isSingleView();
 		this.panelview.removePanel(panel);
 		const [panelItem] = this.panelItems.splice(index, 1);
 		panelItem.disposable.dispose();
 
 		this.updateViewHeaders();
+		if (wasSingleView !== this.isSingleView()) {
+			this.updateTitleArea();
+		}
 	}
 
 	movePanel(from: ViewletPanel, to: ViewletPanel): void {

src/vs/workbench/parts/files/electron-browser/explorerViewlet.ts

@@ -35,8 +35,6 @@ import { IContextMenuService } from 'vs/platform/contextview/browser/contextView
 import { Disposable } from 'vs/base/common/lifecycle';
 import { IWorkbenchContribution } from 'vs/workbench/common/contributions';
 import { IPartService } from 'vs/workbench/services/part/common/partService';
-import { ViewletPanel } from 'vs/workbench/browser/parts/views/panelViewlet';
-
 
 export class ExplorerViewletViewsContribution extends Disposable implements IWorkbenchContribution {
 
@@ -241,16 +239,6 @@ export class ExplorerViewlet extends PersistentViewsViewlet implements IExplorer
 		return <EmptyView>this.getView(EmptyView.ID);
 	}
 
-	public addPanel(panel: ViewletPanel, size: number, index: number): void {
-		super.addPanel(panel, size, index);
-		this.updateTitleArea();
-	}
-
-	public removePanel(panel: ViewletPanel): void {
-		super.removePanel(panel);
-		this.updateTitleArea();
-	}
-
 	public setVisible(visible: boolean): TPromise<void> {
 		this.viewletVisibleContextKey.set(visible);
 		return super.setVisible(visible);