You can reproduce the problem as long as your panel height is bigger enough.
Please see the code below:
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html>
<html>
<head id="Head1" runat="server">
<title></title>
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
ResourceManager.GetInstance().RegisterBeforeClientInitScript("window._start = new Date().getTime();");
var data = new List<object>();
for (int i = 0; i < 500; i++)
{
data.Add(new {
Group = i % 100,
Value1 = "1_" + i,
Value2 = "2_" + i,
Value3 = "3_" + i,
Value4 = "4_" + i,
Value5 = "5_" + i,
Value6 = "6_" + i,
Value7 = "7_" + i,
Value8 = "8_" + i,
Value9 = "9_" + i,
Value10 = "10_" + i,
Value11 = "11_" + i,
Value12 = "12_" + i,
Value13 = "13_" + i,
Value14 = "14_" + i,
Value15 = "15_" + i,
Value16 = "16_" + i,
Value17 = "17_" + i,
Value18 = "18_" + i,
Value19 = "19_" + i,
Value20 = "20_" + i,
Value21 = "21_" + i,
Value22 = "22_" + i,
Value23 = "23_" + i,
Value24 = "24_" + i,
Value25 = "25_" + i,
Value26 = "26_" + i,
Value27 = "27_" + i,
Value28 = "28_" + i,
Value29 = "29_" + i,
Value30 = "30_" + i
});
}
GridPanel1.GetStore().DataSource = data;
GridPanel1.GetStore().DataBind();
}
</script>
</head>
<body>
<form id="Form1" runat="server">
<ext:ResourceManager ID="ResourceManager1" runat="server" />
<ext:GridPanel
ID="GridPanel1"
runat="server"
Width="1024" Height="860"
Title="Grid"
>
<Store>
<ext:Store ID="Store1" runat="server" GroupField="Group" PageSize="1000" >
<Model>
<ext:Model ID="Model1" runat="server">
<Fields>
<ext:ModelField Name="Group" />
<ext:ModelField Name="Value1" />
<ext:ModelField Name="Value2" />
<ext:ModelField Name="Value3" />
<ext:ModelField Name="Value4" />
<ext:ModelField Name="Value5" />
<ext:ModelField Name="Value6" />
<ext:ModelField Name="Value7" />
<ext:ModelField Name="Value8" />
<ext:ModelField Name="Value9" />
<ext:ModelField Name="Value10" />
<ext:ModelField Name="Value11" />
<ext:ModelField Name="Value12" />
<ext:ModelField Name="Value13" />
<ext:ModelField Name="Value14" />
<ext:ModelField Name="Value15" />
<ext:ModelField Name="Value16" />
<ext:ModelField Name="Value17" />
<ext:ModelField Name="Value18" />
<ext:ModelField Name="Value19" />
<ext:ModelField Name="Value20" />
<ext:ModelField Name="Value21" />
<ext:ModelField Name="Value22" />
<ext:ModelField Name="Value23" />
<ext:ModelField Name="Value24" />
<ext:ModelField Name="Value25" />
<ext:ModelField Name="Value26" />
<ext:ModelField Name="Value27" />
<ext:ModelField Name="Value28" />
<ext:ModelField Name="Value29" />
<ext:ModelField Name="Value30" />
</Fields>
</ext:Model>
</Model>
<Sorters>
<ext:DataSorter Property="Group" Direction="ASC" />
</Sorters>
</ext:Store>
</Store>
<VerticalScroller>
<ext:GridPagingScroller ID="GridPagingScroller1" runat="server" ActivePrefetch="false" />
</VerticalScroller>
<ColumnModel ID="ColumnModel1" runat="server">
<Columns>
<ext:RowNumbererColumn ID="RowNumbererColumn1" runat="server" Width="40" Sortable="false" />
<ext:Column ID="Column1" runat="server" Text="Group" DataIndex="Group" />
<ext:SummaryColumn ID="SummaryColumn1" runat="server" Text="Value1" DataIndex="Value1" SummaryType="Count" Flex="1" />
<ext:Column ID="Column2" runat="server" Text="Value2" DataIndex="Value2" Flex="1" />
<ext:Column ID="Column3" runat="server" Text="Value3" DataIndex="Value3" Flex="1" />
<ext:Column ID="Column4" runat="server" Text="Value4" DataIndex="Value4" Flex="1" />
<ext:Column ID="Column5" runat="server" Text="Value5" DataIndex="Value5" Flex="1" />
<ext:Column ID="Column6" runat="server" Text="Value6" DataIndex="Value6" Flex="1" />
<ext:Column ID="Column7" runat="server" Text="Value7" DataIndex="Value7" Flex="1" />
<ext:Column ID="Column8" runat="server" Text="Value8" DataIndex="Value8" Flex="1" />
<ext:Column ID="Column9" runat="server" Text="Value9" DataIndex="Value9" Flex="1" />
<ext:Column ID="Column10" runat="server" Text="Value10" DataIndex="Value10" Flex="1" />
<ext:Column ID="Column11" runat="server" Text="Value11" DataIndex="Value11" Flex="1" />
<ext:Column ID="Column12" runat="server" Text="Value12" DataIndex="Value12" Flex="1" />
<ext:Column ID="Column13" runat="server" Text="Value13" DataIndex="Value13" Flex="1" />
<ext:Column ID="Column14" runat="server" Text="Value14" DataIndex="Value14" Flex="1" />
<ext:Column ID="Column15" runat="server" Text="Value15" DataIndex="Value15" Flex="1" />
<ext:Column ID="Column16" runat="server" Text="Value16" DataIndex="Value16" Flex="1" />
<ext:Column ID="Column17" runat="server" Text="Value17" DataIndex="Value17" Flex="1" />
<ext:Column ID="Column18" runat="server" Text="Value18" DataIndex="Value18" Flex="1" />
<ext:Column ID="Column19" runat="server" Text="Value19" DataIndex="Value19" Flex="1" />
<ext:Column ID="Column20" runat="server" Text="Value20" DataIndex="Value20" Flex="1" />
<ext:Column ID="Column21" runat="server" Text="Value21" DataIndex="Value21" Flex="1" />
<ext:Column ID="Column22" runat="server" Text="Value22" DataIndex="Value22" Flex="1" />
<ext:Column ID="Column23" runat="server" Text="Value23" DataIndex="Value23" Flex="1" />
<ext:Column ID="Column24" runat="server" Text="Value24" DataIndex="Value24" Flex="1" />
<ext:Column ID="Column25" runat="server" Text="Value25" DataIndex="Value25" Flex="1" />
<ext:Column ID="Column26" runat="server" Text="Value26" DataIndex="Value26" Flex="1" />
<ext:Column ID="Column27" runat="server" Text="Value27" DataIndex="Value27" Flex="1" />
<ext:Column ID="Column28" runat="server" Text="Value28" DataIndex="Value28" Flex="1" />
<ext:Column ID="Column29" runat="server" Text="Value29" DataIndex="Value29" Flex="1" />
<ext:Column ID="Column30" runat="server" Text="Value30" DataIndex="Value30" Flex="1" />
</Columns>
</ColumnModel>
<Features>
<ext:GroupingSummary ID="GroupingView1" runat="server"
HideGroupedHeader
="false" EnableGroupingMenu="false"
GroupHeaderTpl='Group {[values.rows[0].data.Group]}' />
<ext:GridFilters runat="server" ID="GridFilters1">
<Filters>
<ext:StringFilter DataIndex='Value2' />
<ext:StringFilter DataIndex='Value3' />
<ext:StringFilter DataIndex='Value4' />
<ext:StringFilter DataIndex='Value5' />
<ext:StringFilter DataIndex='Value6' />
</Filters>
</ext:GridFilters>
</Features>
<View>
<ext:GridView ID="GridView1" runat="server" TrackOver="false" >
<Listeners>
<%--<Refresh Handler="alert((new Date().getTime() - window._start).toString());" />--%>
</Listeners>
</ext:GridView>
</View>
</ext:GridPanel>
</form>
</body>
</html>