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 
},
        });
      });
      this._setEventListeners(
        selectElement,
        [
          ["focus", "Focus"],
          ["blur", "Blur"],
          ["mousedown", "Mouse Down"],
          ["mouseenter", "Mouse Enter"],
          ["mouseleave", "Mouse Exit"],
          ["mouseup", "MouseUp"],
        ],
        event => event.target.checked
      );
    } else {
      selectElement.addEventListener("input", function (event) {
        storage.setValue(id, { value: getValue(event) });
      });
    }
    this.container.appendChild(selectElement);
    return this.container;
  }
}
class PopupAnnotationElement extends AnnotationElement {
  constructor(parameters) {
    const isRenderable = !!(parameters.data.title || parameters.data.contents);
    super(parameters, { isRenderable });
  }
  render() {