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 
function compare(a, b) {
      return a - b;
    }
    
    const orderedX = [rect1[0], rect1[2], rect2[0], rect2[2]].sort(compare);
    const orderedY = [rect1[1], rect1[3], rect2[1], rect2[3]].sort(compare);
    const result = [];
    rect1 = Util.normalizeRect(rect1);
    rect2 = Util.normalizeRect(rect2);
    
    if (
      (orderedX[0] === rect1[0] && orderedX[1] === rect2[0]) ||
      (orderedX[0] === rect2[0] && orderedX[1] === rect1[0])
    ) {
      
      result[0] = orderedX[1];