/* Identify rows where the primary parent is off screen.  Assign
  ** each to a rail and draw descenders downward.
  **
  ** Strive to put the main branch (usually "trunk") on far left.
  */
  zMainBranch = db_get("main-branch", 0);
  zTrunk = persistBranchName(p, zMainBranch);
  fossil_free(zMainBranch);
  for(i=0; i<2; i++){
    for(pRow=p->pLast; pRow; pRow=pRow->pPrev){
      if( i==0 && pRow->zBranch!=zTrunk ) continue;
      if( pRow->iRail>=0 ) continue;
      if( pRow->isDup ) continue;
      if( pRow->nParent<0 ) continue;
      if( pRow->nParent==0 || hashFind(p,pRow->aParent[0])==0 ){