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 
DS: "DidSave",
  WP: "WillPrint",
  DP: "DidPrint",
};
const PageActionEventType = {
  O: "PageOpen",
  C: "PageClose",
};
const StreamType = {
  UNKNOWN: "UNKNOWN",
  FLATE: "FLATE",
  LZW: "LZW",
  DCT: "DCT",
  JPX: "JPX",
  JBIG: "JBIG",
  A85: "A85",
  AHX: "AHX",
  CCF: "CCF",
  RLX: "RLX", 
};
const FontType = {
  UNKNOWN: "UNKNOWN",
  TYPE1: "TYPE1",
  TYPE1C: "TYPE1C",
  CIDFONTTYPE0: "CIDFONTTYPE0",
  CIDFONTTYPE0C: "CIDFONTTYPE0C",
  TRUETYPE: "TRUETYPE",
  CIDFONTTYPE2: "CIDFONTTYPE2",
  TYPE3: "TYPE3",
  OPENTYPE: "OPENTYPE",