Hi,

Thank you for a sample, I reproduced that in Firefox. All other browsers: IE, Safari, Chrome, Opera work fine.

The logic of splitter is based on assumption that size of its outer <div> (which has overflow:hidden) matches with actual size of control. All calculations of pane layout are based on that assumption. I tested behavior of <table> in Firefox and found that is rather strange. The size of its <td> increases if content of its child is modified/increased by javascript. Even that content is located in <div> with hidden overflow, but Firefox seems to ignore that attribute and increases overall width of table.
To me it looks like a bug in Firefox.

Attempts to get around that specific issue of Firefox in logic of splitter worked in a specific sample, but failed for a general case.
I advise to remove splitter from table. If that is not possible for application, then the best I can do, is to suggest following work around:

1. Process before and after expand events.
2. Within before expand event, check if width is % and it is Firefox and replace % width by px.
3. Within after expand event, check if width was modified and restore old width.

Below is implementation
<script type="text/javascript">
function expandedEvt(splitter, evtArgs)
{
 var width = splitter._widthToRestore;
 if(width)
  splitter.get_element().style.width = width;
}
function expandingEvt(splitter, evtArgs)
{
 var elem = splitter.get_element();
 var width = elem.style.width, oW = elem.offsetWidth;
 if(width.indexOf('%') < 0 || oW < 10 || !$util.IsFireFox)
  return;
 elem.style.width = (oW - 4) + 'px';
 splitter._widthToRestore = width;
}
</script>

<ig:WebSplitter ID="WebSplitter1" runat="server" Height="200px" Width="100%">
 ...
 <ClientEvents Expanded="expandedEvt" Expanding="expandingEvt" />
</ig:WebSplitter>

The problem occures when the WebSpliter is inside table. Please try this :

        <table class="style1">
            <tr>
                <td>
                    <ig:WebSplitter ID="WebSplitter1" runat="server" Height="200px" Width="100%">
                        <panes>
                            <ig:SplitterPane runat="server" CollapsedDirection="PreviousPane"
                                MinSize="150px">
                            </ig:SplitterPane>
                            <ig:SplitterPane runat="server">
                            </ig:SplitterPane>
                        </panes>
                    </ig:WebSplitter>
                </td>
            </tr>
        </table>