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 
buttonWidgetRef,
        pdfManagerMock,
        idFactoryMock
      )
        .then(annotation => {
          const annotationStorage = {};
          annotationStorage[annotation.data.id] = { value: true };
          return Promise.all([
            annotation,
            annotation.getOperatorList(
              partialEvaluator,
              task,
              false,
              annotationStorage
            ),
          ]);
        }, done.fail)
        .then(([annotation, opList]) => {
          expect(opList.argsArray.length).toEqual(3);
          expect(opList.fnArray).toEqual([
            OPS.beginAnnotation,
            OPS.setFillRGBColor,
            OPS.endAnnotation,
          ]);
          expect(opList.argsArray[1]).toEqual(
            new Uint8ClampedArray([26, 51, 76])
          );
          return annotation;
        }, done.fail)
        .then(annotation => {