ListGrid stuck in "Data Loading" when fetchData is called before the first finishes

SmartClient Version: v9.0p_2013-11-03/PowerEdition
Browsers: IE 8-10, Chrome 32, probably others

I have a ListGrid that fetches data from a DataSource based on user interaction with other components. Sometimes the user initiates another fetchData before the previous call to fetchData has completed. Whenever this happens the grid gets stuck in a "Loading Data" state.

There are no errors that occur on the server side and I can see the data coming back to the client just fine. The fetchData callback method for both fetchData requests also gets called without a problem. But the ListGrid never displays the rows. It just continues showing the "Loading Data" message.

You can duplicate this behavior by modifying the code in the example:
http://www.smartclient.com/#databoundFetch

isc.ListGrid.create({
ID: "countryList",
width:500, height:224, alternateRecordStyles:true,
dataSource: worldDS,
// display a subset of fields from the datasource
fields:[
{name:"countryCode"},
{name:"countryName"},
{name:"capital"},
{name:"continent"}
],
sortFieldNum: 1, // sort by countryName
dataPageSize: 50,
drawAheadRatio: 4
})

isc.IButton.create({
left:1, top:240, width:160,
title:"Fetch Double",
click: function() {
countryList.fetchData(
{continent:'Europe'},
function(dsResponse, data, dsRequest){
isc.say('callback 1 '+dsRequest.clientContext.getDataSource().ID);
},
{clientContext: countryList}
);
setTimeout(function() {
countryList.fetchData(
{continent:'Africa'},
function(dsResponse, data, dsRequest){
isc.say('callback 2 '+dsRequest.clientContext.getDataSource().ID);
},
{clientContext: countryList}
);
}, 100);
}
})

Any suggestions on why this might be happening and the best way to prevent the grid from getting stuck in the "Data Loading" state?