Hello!
It seems to be a bug of TableLayout in Ext JS. We submitted a bug report to Sencha. Here is ticket:
https://github.com/extnet/Ext.NET/issues/380
For now, try to use HBox layout, like in the following sample:
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<html>
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<ext:ResourceManager ID="ResourceManager1" runat="server" ScriptMode="Development">
</ext:ResourceManager>
<ext:Viewport ID="ViewPort" runat="server" AutoScroll="True" Layout="FitLayout">
<Items>
<ext:FormPanel runat="server" ID="FormPanelVacation" AutoScroll="True" Layout="HBox">
<Items>
<ext:Container Width="300">
<Items>
<ext:Button runat="server" ID="ShowButton" Text="Show" Handler="App.FieldSetOtherVacations.show();" />
<ext:Button runat="server" ID="HideButton" Text="Hide" Handler="App.FieldSetOtherVacations.hide();" />
</Items>
</ext:Container>
<ext:FieldSet runat="server" ID="FieldSetOtherVacations" Collapsible="True" Width="200" Title="Other Vacations" HideMode="Display">
<LayoutConfig>
<ext:TableLayoutConfig Columns="2" ItemCls="panel-table-items">
</ext:TableLayoutConfig>
</LayoutConfig>
<Items>
<ext:TextField runat="server" />
</Items>
</ext:FieldSet>
</Items>
</ext:FormPanel>
</Items>
</ext:Viewport>
</form>
</body>
</html>