以下のコードをグリッド生成後に呼び出します。

function initialized(grid) {
  if (!grid) return;

  let downX = 0;
  let drag = null;
  let onUp = null;

  const host = grid.hostElement;

  host.addEventListener(
    'mousedown',
    function (e) {
      drag = null;
      downX = e.pageX;

      onUp = function (ev) {
        if (drag) {
          const col = grid.columns.getColumn(drag.col);

          drag.desired = Math.max(
            col.minWidth || 0,
            Math.round(drag.startWidth + (ev.pageX - downX))
          );
        }

        window.removeEventListener('mouseup', onUp, true);
        onUp = null;
      };

      window.addEventListener('mouseup', onUp, true);
    },
    true
  );

  grid.resizingColumn.addHandler(function (s, e) {
    if (e.panel !== s.cells || !s.columns.isFrozen(e.col)) {
      return;
    }

    if (!drag) {
      drag = {
        col: e.col,
        startWidth: s.columns.getColumn(e.col).renderWidth
      };
    }

    const data = e.data;
    if (data && data.maxSize != null) {
      data.maxSize = null;
    }
  });

  grid.resizedColumn.addHandler(function (s, e) {
    if (e.panel !== s.cells || !drag || e.col !== drag.col) {
      return;
    }

    const d = drag;

    queueMicrotask(function () {
      if (d.desired == null) {
        return;
      }

      const col = s.columns.getColumn(d.col);

      if (col.width !== d.desired) {
        col.width = d.desired;
      }
    });
  });
}