_handleZIndex({ phase }) {
    if (this.placementMode !== 'local') {
      return;
    }

    if (phase === 'setup') {
      const zIndexNumber = Number(getComputedStyle(this.contentNode).zIndex);
      if (zIndexNumber < 1 || Number.isNaN(zIndexNumber)) {
        this.contentNode.style.zIndex = '1';
      }
    }
  }
Display local overlays on top of elements with no z-index that appear later in the DOM
@param {{ phase: OverlayPhase }} config
@protected