src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/setActive
class EditorGroupModel:	setActive(candidate: EditorInput | undefined): EditorInput | undefined {
		let result: EditorInput | undefined = undefined;

		if (!candidate) {
			this.setGroupActive();
		} else {
			result = this.setEditorActive(candidate);
		}

		return result;
	}