https://github.com/mozilla/pdf.js/issues/6397#issuecomment-164954332 Would have been a lot easier to find had there been an assertion checking that string16 didn't go out of bounds of the 16bit number.

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];