src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/isSticky
class EditorGroupModel:	isSticky(candidateOrIndex: EditorInput | number): boolean {
		if (this.sticky < 0) {
			return false; // no sticky editor
		}

		let index: number;
		if (typeof candidateOrIndex === 'number') {
			index = candidateOrIndex;
		} else {
			index = this.indexOf(candidateOrIndex);
		}

		if (index < 0) {
			return false;
		}

		return index <= this.sticky;
	}