static/ext/src/ux/dd/CellFieldDropZone.js
function(target, dd, e, dragData) {
        delete this.dropOK;
        if (!target) {
            return;
        }

//      Check that a field is being dragged.
        var f = dragData.field;
        if (!f) {
            return;
        }

//      Check whether the data type of the column being dropped on accepts the
//      dragged field type. If so, set dropOK flag, and highlight the target node.
        var type = target.record.fields.get(target.fieldName).type,
            types = Ext.data.Types;
        switch(type){
            case types.FLOAT:
            case types.INT:
                if (!f.isXType('numberfield')) {
                    return;
                }
                break;
            case types.DATE:
                if (!f.isXType('datefield')) {
                    return;
                }
                break;
            case types.BOOL:
                if (!f.isXType('checkbox')) {
                    return;
                }
        }
        this.dropOK = true;
        Ext.fly(target.node).addCls('x-drop-target-active');
    }
function(target, dd, e, dragData) {
        delete this.dropOK;
        if (!target) {
            return;
        }

//      Check that a field is being dragged.
        var f = dragData.field;
        if (!f) {
            return;
        }

//      Check whether the data type of the column being dropped on accepts the
//      dragged field type. If so, set dropOK flag, and highlight the target node.
        var type = target.record.fields.get(target.fieldName).type,
            types = Ext.data.Types;
        switch(type){
            case types.FLOAT:
            case types.INT:
                if (!f.isXType('numberfield')) {
                    return;
                }
                break;
            case types.DATE:
                if (!f.isXType('datefield')) {
                    return;
                }
                break;
            case types.BOOL:
                if (!f.isXType('checkbox')) {
                    return;
                }
        }
        this.dropOK = true;
        Ext.fly(target.node).addCls('x-drop-target-active');
    }
On Node enter, see if it is valid for us to drop the field on that type of column.