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 
annotationDict.set("Type", Name.get("Annot"));
        annotationDict.set("Subtype", Name.get("Link"));
        const xref = new XRefMock();
        const idFactory = createIdFactory( 0);
        const annotation1 = AnnotationFactory.create(
          xref,
          annotationDict,
          pdfManagerMock,
          idFactory
        ).then(({ data }) => {
          expect(data.annotationType).toEqual(AnnotationType.LINK);
          expect(data.id).toEqual("annot_p0_1");
        });
        const annotation2 = AnnotationFactory.create(
          xref,
          annotationDict,
          pdfManagerMock,
          idFactory
        ).then(({ data }) => {
          expect(data.annotationType).toEqual(AnnotationType.LINK);
          expect(data.id).toEqual("annot_p0_2");
        });
        Promise.all([annotation1, annotation2]).then(done, done.fail);
      }
    );
    it("should handle missing /Subtype", function (done) {
      const annotationDict = new Dict();
      annotationDict.set("Type", Name.get("Annot"));
      const annotationRef = Ref.get(1, 0);
      const xref = new XRefMock([{ ref: annotationRef, data: annotationDict }]);