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 
let container = this.container;
    if (this.quadrilaterals) {
      trigger = trigger || this.quadrilaterals;
      container = this.quadrilaterals[0];
    }
    
    if (!trigger) {
      trigger = document.createElement("div");
      trigger.style.height = container.style.height;
      trigger.style.width = container.style.width;
      container.appendChild(trigger);
    }
    const popupElement = new PopupElement({
      container,
      trigger,
      color: data.color,
      title: data.title,
      modificationDate: data.modificationDate,
      contents: data.contents,
      hideWrapper: true,
    });
    const popup = popupElement.render();
    
    popup.style.left = container.style.width;
    container.appendChild(popup);
  }
  /**
   * Render the quadrilaterals of the annotation.
   *