code: function _getViewportExpressions (rootExpressions) {
      const viewportExpressions = [];
  
      function dfs (expr) {
          if (expr._isViewport) {
              viewportExpressions.push(expr);
          } else {
              expr._getChildren().map(dfs);
          }
      }
  
      rootExpressions.map(dfs);
  
      return viewportExpressions;
  }
docs: Perform a DFS (Depth First Search) through the expression tree collecting all viewport expressions.
  This is a performance optimization to avoid doing DFS at each feature iteration