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 
buttonWidgetDict.set("AP", appearanceStatesDict);
      const buttonWidgetRef = Ref.get(124, 0);
      const xref = new XRefMock([
        { ref: buttonWidgetRef, data: buttonWidgetDict },
      ]);
      const task = new WorkerTask("test print");
      partialEvaluator.options = { ignoreErrors: true };
      AnnotationFactory.create(
        xref,
        buttonWidgetRef,
        pdfManagerMock,
        idFactoryMock
      )
        .then(annotation => {
          const annotationStorage = {};
          annotationStorage[annotation.data.id] = { value: true };
          return annotation.getOperatorList(
            partialEvaluator,
            task,
            false,
            annotationStorage
          );
        })
        .then(opList => {
          expect(opList.argsArray.length).toEqual(5);
          expect(opList.fnArray).toEqual([
            OPS.beginAnnotation,
            OPS.dependency,
            OPS.setFont,
            OPS.showText,