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 
const annotation = new Annotation({ dict, ref });
      annotation.setContents(undefined);
      expect(annotation.contents).toEqual("");
    });
    it("should set and get a valid modification date", function () {
      const annotation = new Annotation({ dict, ref });
      annotation.setModificationDate("D:20190422");
      expect(annotation.modificationDate).toEqual("D:20190422");
    });
    it("should not set and get an invalid modification date", function () {
      const annotation = new Annotation({ dict, ref });
      annotation.setModificationDate(undefined);
      expect(annotation.modificationDate).toEqual(null);
    });
    it("should set and get flags", function () {
      const annotation = new Annotation({ dict, ref });
      annotation.setFlags(13);
      expect(annotation.hasFlag(AnnotationFlag.INVISIBLE)).toEqual(true);
      expect(annotation.hasFlag(AnnotationFlag.NOZOOM)).toEqual(true);
      expect(annotation.hasFlag(AnnotationFlag.PRINT)).toEqual(true);
      expect(annotation.hasFlag(AnnotationFlag.READONLY)).toEqual(false);
      expect(annotation.hasFlag(AnnotationFlag.HIDDEN)).toEqual(false);
    });
    it("should be viewable and not printable by default", function () {
      const annotation = new Annotation({ dict, ref });
      expect(annotation.viewable).toEqual(true);
      expect(annotation.printable).toEqual(false);
    });
    it("should set and get a valid rectangle", function () {
      const annotation = new Annotation({ dict, ref });