keep active tab visible when resizing

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

@@ -1278,10 +1278,15 @@ export class SideBySideEditorControl implements ISideBySideEditorControl, IVerti
 			}
 		});
 
-		// Layout active editors
+		// Layout visible editors
 		POSITIONS.forEach(position => {
 			this.layoutEditor(position);
 		});
+
+		// Layout title controls
+		POSITIONS.forEach(position => {
+			this.titleAreaControl[position].layout();
+		});
 	}
 
 	private layoutEditor(position: Position): void {

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

@@ -30,6 +30,7 @@ import {TitleControl} from 'vs/workbench/browser/parts/editor/titleControl';
 export class TabsTitleControl extends TitleControl {
 	private titleContainer: Builder;
 	private tabsContainer: Builder;
+	private activeTab: Builder;
 
 	private groupActionsToolbar: ToolBar;
 	private tabActionBars: ActionBar[];
@@ -140,8 +141,6 @@ export class TabsTitleControl extends TitleControl {
 			this.tabActionBars.pop().dispose();
 		}
 
-		let activeTab: HTMLElement;
-
 		// Add a tab for each opened editor
 		this.context.getEditors().forEach(editor => {
 			const isPinned = group.isPinned(editor);
@@ -163,7 +162,7 @@ export class TabsTitleControl extends TitleControl {
 				// Active state
 				if (isActive) {
 					tab.addClass('active');
-					activeTab = tab.getHTMLElement();
+					this.activeTab = $(tab);
 				} else {
 					tab.removeClass('active');
 				}
@@ -194,7 +193,16 @@ export class TabsTitleControl extends TitleControl {
 			});
 		});
 
+		this.layout();
+	}
+
+	public layout(): void {
+		if (!this.activeTab) {
+			return;
+		}
+
 		// Always reveal the active one
+		const activeTab = this.activeTab.getHTMLElement();
 		const container = this.tabsContainer.getHTMLElement();
 		const containerWidth = container.offsetWidth;
 		const containerScrollPosX = container.scrollLeft;

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

@@ -43,6 +43,7 @@ export interface ITitleAreaControl {
 	allowDragging(element: HTMLElement): boolean;
 	create(parent: Builder): void;
 	refresh(): void;
+	layout(): void;
 	dispose(): void;
 }
 
@@ -95,6 +96,10 @@ export abstract class TitleControl {
 
 	public abstract refresh();
 
+	public layout(): void {
+		// Subclasses can opt in to react on layout
+	}
+
 	public allowDragging(element: HTMLElement): boolean {
 		return !DOM.findParentWithClass(element, 'monaco-action-bar', 'one-editor-container');
 	}