message: Fix the drop position calculation
diff: @@ -524,15 +524,16 @@ export class TabWidget extends ThemeableElementBase {
  private _pointToTabIndex(ev: DragEvent): number {
  // Figure out which tabs the drop indicator should appear in between.
  - const rect = this.getBoundingClientRect();
  - const pointXInPageCoords = ev.pageX - rect.left;
  -
  + const widgetRect = this.getBoundingClientRect();
  + const pointXInPageCoords = ev.pageX - widgetRect.left;
  const tabBar = this._getTabbar();
  - let index = 0;
  const childElements = DomUtils.toArray(tabBar.children).filter(kid => kid.classList.contains(CLASS_TAB));
  +
  + let index = 0;
  for (const kid of childElements) {
  - const rect = kid.getBoundingClientRect();
  - if (pointXInPageCoords <= rect.left + rect.width/2) {
  + const kidRect = kid.getBoundingClientRect();
  + const midPoint = kidRect.left - widgetRect.left + kidRect.width/2;
  + if (pointXInPageCoords <= midPoint) {
  break;
  }
  index++;
  