message: get computed value for z-index of parentDiv for viewport
diff: @@ -1742,7 +1742,8 @@ export class ScreenViewport extends Viewport {
  while (parentDiv.lastChild)
  parentDiv.removeChild(parentDiv.lastChild);
  - const parentZ = parseInt(parentDiv.style.zIndex || "0", 10);
  + // get the (computed) z-index value of the parent, as an integer.
  + const parentZ = parseInt(window.getComputedStyle(parentDiv).zIndex || "0", 10);
  addChild(canvas, parentZ + 1);
  @@ -1909,9 +1910,6 @@ export class OffScreenViewport extends Viewport {
  this.changeView(this.view);
  }
  - /** @hidden */
  public prepareDecorations(_decorations: Decorations): void { }
  -
  - /** @hidden */
  public decorate(_context: DecorateContext): void { }
  }
  