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 
helvDict.set("Type", Name.get("Font"));
      helvDict.set("Subtype", Name.get("Type1"));
      const gothDict = new Dict();
      gothDict.set("BaseFont", Name.get("MSGothic"));
      gothDict.set("Type", Name.get("Font"));
      gothDict.set("Subtype", Name.get("Type0"));
      gothDict.set("Encoding", Name.get("UniJIS-UTF16-H"));
      gothDict.set("Name", Name.get("MSGothic"));
      const cidSysInfoDict = new Dict();
      cidSysInfoDict.set("Ordering", "Japan1");
      cidSysInfoDict.set("Registry", "Adobe");
      cidSysInfoDict.set("Supplement", "5");
      const fontDescriptorDict = new Dict();
      fontDescriptorDict.set("FontName", Name.get("MSGothic"));
      fontDescriptorDict.set("CapHeight", "680");
      const gothDescendantDict = new Dict();
      gothDescendantDict.set("BaseFont", Name.get("MSGothic"));
      gothDescendantDict.set("CIDSystemInfo", cidSysInfoDict);
      gothDescendantDict.set("Subtype", Name.get("CIDFontType2"));
      gothDescendantDict.set("Type", Name.get("Font"));
      gothDescendantDict.set("FontDescriptor", fontDescriptorDict);
      gothDict.set("DescendantFonts", [gothDescendantDict]);
      const helvRef = Ref.get(314, 0);
      const gothRef = Ref.get(159, 0);
      helvRefObj = { ref: helvRef, data: helvDict };
      gothRefObj = { ref: gothRef, data: gothDict };
      const resourceDict = new Dict();
      const fontDict = new Dict();
      fontDict.set("Helv", helvRef);
      resourceDict.set("Font", fontDict);