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 
annotationDict.set("C", [0, 0, 1]);
        const replyRef = Ref.get(820, 0);
        const replyDict = new Dict();
        replyDict.set("Type", Name.get("Annot"));
        replyDict.set("Subtype", Name.get("Text"));
        replyDict.set("IRT", annotationRef);
        replyDict.set("RT", Name.get("Group"));
        replyDict.set("T", "Reply Title");
        replyDict.set("Contents", "Reply Text");
        replyDict.set("M", "D:20190523");
        replyDict.set("C", [0.4]);
        const pupRef = Ref.get(821, 0);
        const pupDict = new Dict();
        pupDict.set("Type", Name.get("Annot"));
        pupDict.set("Subtype", Name.get("Popup"));
        pupDict.set("T", "Wrong Title");
        pupDict.set("Contents", "Wrong Text");
        pupDict.set("Parent", replyRef);
        pupDict.set("M", "D:20190623");
        pupDict.set("C", [0.8]);
        replyDict.set("Popup", pupRef);
        const xref = new XRefMock([
          { ref: annotationRef, data: annotationDict },
          { ref: replyRef, data: replyDict },
          { ref: pupRef, data: pupDict },
        ]);
        annotationDict.assignXref(xref);
        pupDict.assignXref(xref);
        replyDict.assignXref(xref);
        AnnotationFactory.create(