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 
const NullCharactersRegExp = /\x00/g;
/**
 * @param {string} str
 */
function removeNullCharacters(str) {
  if (typeof str !== "string") {
    warn("The argument for removeNullCharacters must be a string.");
    return str;
  }
  return str.replace(NullCharactersRegExp, "");
}
function bytesToString(bytes) {
  assert(
    bytes !== null && typeof bytes === "object" && bytes.length !== undefined,
    "Invalid argument for bytesToString"
  );
  const length = bytes.length;
  const MAX_ARGUMENT_COUNT = 8192;
  if (length < MAX_ARGUMENT_COUNT) {
    return String.fromCharCode.apply(null, bytes);
  }
  const strBuf = [];
  for (let i = 0; i < length; i += MAX_ARGUMENT_COUNT) {
    const chunkEnd = Math.min(i + MAX_ARGUMENT_COUNT, length);
    const chunk = bytes.subarray(i, chunkEnd);
    strBuf.push(String.fromCharCode.apply(null, chunk));
  }
  return strBuf.join("");
}
function stringToBytes(str) {