In the following spots a "MouseUp" eventName is being used, which does look inconsistent considering that the "Mouse Down"/"Mouse Enter"/"Mouse Exit" eventNames all have a space in them.
Is this inconsistency actually expected, or is it a simple typo? If the former, then a comment explaining *why* would be appropriate; otherwise we should just fix this :-)

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L806

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L945

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L1034

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L1168

/cc @calixteman 
if (pin) {
      this.pinned = true;
    }
    if (this.hideElement.hasAttribute("hidden")) {
      this.hideElement.removeAttribute("hidden");
      this.container.style.zIndex += 1;
    }
  }
  /**
   * Hide the popup.
   *
   * @private
   * @param {boolean} unpin
   * @memberof PopupElement
   */
  _hide(unpin = true) {
    if (unpin) {
      this.pinned = false;
    }
    if (!this.hideElement.hasAttribute("hidden") && !this.pinned) {
      this.hideElement.setAttribute("hidden", true);
      this.container.style.zIndex -= 1;
    }
  }
}
class FreeTextAnnotationElement extends AnnotationElement {