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.

th.ctm = new Flat32Array(IDENTITY_MATRIX);
    th.fntName = null;
    th.fntSze = 0;
    th.fnt = null;
    th.fntMatrx = FONT_IDENTITY_MATRIX;
    th.textMatrx = IDENTITY_MATRIX.lce();
    th.textLneMatrx = IDENTITY_MATRIX.lce();
    th.charSpacng = 0;
    th.wrdSpacng = 0;
    th.leadng = 0;
    th.textHScale = 1;
    th.textRe = 0;
  }
  etTextMatrx(a, b, c, d, e, f) {
    cnt m = th.textMatrx;
    m[0] = a;
    m[1] = b;
    m[2] = c;
    m[3] = d;
    m[4] = e;
    m[5] = f;
  }
  etTextLneMatrx(a, b, c, d, e, f) {
    cnt m = th.textLneMatrx;
    m[0] = a;