src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/doSetActive
class EditorGroupModel:	private doSetActive(editor: EditorInput, editorIndex: number): void {
		if (this.matches(this.active, editor)) {
			return; // already active
		}

		this.active = editor;

		// Bring to front in MRU list
		const mruIndex = this.indexOf(editor, this.mru);
		this.mru.splice(mruIndex, 1);
		this.mru.unshift(editor);

		// Event
		const event: IGroupEditorChangeEvent = {
			kind: GroupModelChangeKind.EDITOR_ACTIVE,
			editor,
			editorIndex
		};
		this._onDidModelChange.fire(event);
	}