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 d1 = b1 >> 2,
        d2 = ((b1 & 3) << 4) | (b2 >> 4);
      const d3 = i + 1 < ii ? ((b2 & 0xf) << 2) | (b3 >> 6) : 64;
      const d4 = i + 2 < ii ? b3 & 0x3f : 64;
      buffer += digits[d1] + digits[d2] + digits[d3] + digits[d4];
    }
    return buffer;
  };
})();
const XMLEntities = {
   0x3c: "&lt;",
   0x3e: "&gt;",
   0x26: "&amp;",
   0x22: "&quot;",
   0x27: "&apos;",
};
function encodeToXmlString(str) {
  const buffer = [];
  let start = 0;
  for (let i = 0, ii = str.length; i < ii; i++) {
    const char = str.codePointAt(i);
    if (0x20 <= char && char <= 0x7e) {
      
      const entity = XMLEntities[char];
      if (entity) {
        if (start < i) {
          buffer.push(str.substring(start, i));
        }
        buffer.push(entity);
        start = i + 1;