message: take bounding box for positioning calculation
diff: @@ -38,23 +38,10 @@ export default class MessageComposerFormatBar extends React.PureComponent {
  showAt(selectionRect) {
  this._formatBarRef.classList.add("mx_MessageComposerFormatBar_shown");
  - let leftOffset = 0;
  - let node = this._formatBarRef;
  - while (node.offsetParent) {
  - node = node.offsetParent;
  - leftOffset += node.offsetLeft;
  - }
  -
  - let topOffset = 0;
  - node = this._formatBarRef;
  - while (node.offsetParent) {
  - node = node.offsetParent;
  - topOffset += node.offsetTop;
  - }
  -
  - this._formatBarRef.style.left = `${selectionRect.left - leftOffset}px`;
  + const parentRect = this._formatBarRef.parentElement.getBoundingClientRect();
  + this._formatBarRef.style.left = `${selectionRect.left - parentRect.left}px`;
  // 12 is half the height of the bar (e.g. to center it) and 16 is an offset that felt ok.
  - this._formatBarRef.style.top = `${selectionRect.top - topOffset - 16 - 12}px`;
  + this._formatBarRef.style.top = `${selectionRect.top - parentRect.top - 16 - 12}px`;
  }
  hide() {
  