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 
it("should accept comb fields with a maximum length", function (done) {
      textWidgetDict.set("MaxLen", 20);
      textWidgetDict.set("Ff", AnnotationFieldFlag.COMB);
      const textWidgetRef = Ref.get(46, 0);
      const xref = new XRefMock([{ ref: textWidgetRef, data: textWidgetDict }]);
      AnnotationFactory.create(
        xref,
        textWidgetRef,
        pdfManagerMock,
        idFactoryMock
      ).then(({ data }) => {
        expect(data.annotationType).toEqual(AnnotationType.WIDGET);
        expect(data.comb).toEqual(true);
        done();
      }, done.fail);
    });
    it("should only accept comb fields when the flags are valid", function (done) {
      const invalidFieldFlags = [
        AnnotationFieldFlag.MULTILINE,
        AnnotationFieldFlag.PASSWORD,
        AnnotationFieldFlag.FILESELECT,
      ];
      
      
      let flags =
        AnnotationFieldFlag.COMB +
        AnnotationFieldFlag.MULTILINE +
        AnnotationFieldFlag.PASSWORD +
        AnnotationFieldFlag.FILESELECT;
      let promise = Promise.resolve();