Hello jouin,

I am just checking have you been able to resolve your issue? If you still need any assistance on the matter do not hesitate to ask.

Hello jouin ,

I have been looking into your issue and can suggest you try the following snippet in order to collapse the pane:

<script type="text/javascript">

    function collapsePane() {

        var splitter = $find("WebSplitter1");

        var a = splitter.getPaneAt(1).set_collapsed(true);

    }

</script>

For additional reference you can check the attached sample.

If you need any further assistance on the matter please do not hesitate to ask.