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 
expect(radioData.ref).toEqual(Ref.get(123, 0));
          expect(radioData.data).toEqual(
            "123 0 obj\n" +
              "<< /Type /Annot /Subtype /Widget /FT /Btn /Ff 32768 " +
              "/AP << /N << /Checked 314 0 R /Off 271 0 R>>>> " +
              "/Parent 456 0 R /AS /Checked /M (date)>>\nendobj\n"
          );
          expect(parentData.ref).toEqual(Ref.get(456, 0));
          expect(parentData.data).toEqual(
            "456 0 obj\n<< /Kids [123 0 R] /V /Checked>>\nendobj\n"
          );
          done();
        })
        .catch(done.fail);
    });
    it("should save nothing", function (done) {
      const buttonWidgetRef = Ref.get(124, 0);
      const xref = new XRefMock([
        { ref: buttonWidgetRef, data: buttonWidgetDict },
      ]);
      const task = new WorkerTask("test save");
      AnnotationFactory.create(
        xref,
        buttonWidgetRef,
        pdfManagerMock,
        idFactoryMock
      )
        .then(annotation => {
          return annotation.save(partialEvaluator, task, {});
        })