要在AG-Grid React中自定义浮动过滤器下拉菜单并将其放置在网格的后面，可以按照以下步骤进行操作：

1. 创建一个自定义浮动过滤器组件，例如CustomFloatingFilter.js：

import React, { useState, useEffect } from 'react';

const CustomFloatingFilter = (props) => {
  const [value, setValue] = useState(null);

  const onChange = (event) => {
    setValue(event.target.value);
    props.onFloatingFilterChanged({ model: value });
  };

  useEffect(() => {
    props.onFloatingFilterChanged({ model: value });
  }, [value]);

  return (
    

      
    
  );
};

export default CustomFloatingFilter;

2. 在使用AG-Grid的地方，将自定义浮动过滤器组件传递给floatingFilterComponent属性：

import React from 'react';
import { AgGridReact } from 'ag-grid-react';
import 'ag-grid-community/dist/styles/ag-grid.css';
import 'ag-grid-community/dist/styles/ag-theme-alpine.css';
import CustomFloatingFilter from './CustomFloatingFilter';

const App = () => {
  const columnDefs = [
    {
      headerName: 'Column 1',
      field: 'col1',
      filter: 'agTextColumnFilter',
      floatingFilterComponent: 'customFloatingFilter',
      floatingFilterComponentParams: {
        suppressFilterButton: true,
      },
    },
    // ... other column definitions
  ];

  const rowData = [
    { col1: 'Value 1' },
    { col1: 'Value 2' },
    // ... other rows
  ];

  return (
    

      

        
      
    
  );
};

export default App;

3. 在样式表中定义自定义浮动过滤器的样式，以使其显示在网格的后面：

.custom-floating-filter {
  position: absolute;
  z-index: 1;
  background-color: white;
  border: 1px solid lightgray;
  padding: 5px;
}

通过这样的设置，您可以实现将自定义浮动过滤器下拉菜单放置在网格的后面。