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 
uncheckedStream.dict = uncheckedAppearanceDict;
      checkedAppearanceDict.set("BBox", [0, 0, 8, 8]);
      checkedAppearanceDict.set("FormType", 1);
      checkedAppearanceDict.set("Matrix", [1, 0, 0, 1, 0, 0]);
      normalAppearanceDict.set("Checked", checkedStream);
      normalAppearanceDict.set("Off", uncheckedStream);
      appearanceStatesDict.set("N", normalAppearanceDict);
      buttonWidgetDict.set("AP", appearanceStatesDict);
      buttonWidgetDict.set("AS", Name.get("Off"));
      const buttonWidgetRef = Ref.get(1249, 0);
      const xref = new XRefMock([
        { ref: buttonWidgetRef, data: buttonWidgetDict },
      ]);
      const task = new WorkerTask("test print");
      AnnotationFactory.create(
        xref,
        buttonWidgetRef,
        pdfManagerMock,
        idFactoryMock
      )
        .then(annotation => {
          const annotationStorage = {};
          annotationStorage[annotation.data.id] = { value: true };
          return Promise.all([
            annotation,
            annotation.getOperatorList(
              partialEvaluator,
              task,
              false,
              annotationStorage