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 
modificationDate.dataset.l10nArgs = JSON.stringify({
        date: dateObject.toLocaleDateString(),
        time: dateObject.toLocaleTimeString(),
      });
      popup.appendChild(modificationDate);
    }
    const contents = this._formatContents(this.contents);
    popup.appendChild(contents);
    if (!Array.isArray(this.trigger)) {
      this.trigger = [this.trigger];
    }
    
    this.trigger.forEach(element => {
      element.addEventListener("click", this._toggle.bind(this));
      element.addEventListener("mouseover", this._show.bind(this, false));
      element.addEventListener("mouseout", this._hide.bind(this, false));
    });
    popup.addEventListener("click", this._hide.bind(this, true));
    wrapper.appendChild(popup);
    return wrapper;
  }
  /**
   * Format the contents of the popup by adding newlines where necessary.
   *
   * @private
   * @param {string} contents
   * @memberof PopupElement
   * @returns {HTMLParagraphElement}
   */
  _formatContents(contents) {