Way we did this on our px-data-grid wrapper

gistfile1.txt

ready() {
  super.ready();
  this._vaadinGrid = this.shadowRoot.querySelector('vaadin-grid');
  // ...
  // Attach scroll listener for styling
  this._boundedScrollListener = this._scrollListener.bind(this);
  this._vaadinGrid.$.outerscroller.addEventListener('scroll', this._boundedScrollListener);
  // ...
}

_scrollListener(e) {
  if (e.target.scrollLeft != 0) {
    this._vaadinGrid.setAttribute('horizontal-offset', 'true');
  } else {
    this._vaadinGrid.removeAttribute('horizontal-offset');
  }
}

// and in theme

[part~="cell"][last-frozen] {
  border-right: 0px solid transparent;
  transition: border 0.3s ease-out;
}

:host([horizontal-offset]) [part~="cell"][last-frozen] {
  border-right: 1px solid var(--px-data-grid-separator-color);
}