The GridPanel plugin example "GridPanel with Local Filtering, Sorting and Paging" (https://examples1.ext.net/#/GridPane...Filters_Local/) contains a button on the PagingToolbar labelled "Find '3m Co'" which calls a server-side method to dynamically set the active filter of the GridPanel's Filter plugin.

If this example is modified by adding a form tag, this button no longer works.
<form runat="server" id="test"></form>

It doesn't seem to matter whether the GridPanel is within the form or not - if a form is present on the page, the button no longer applies the filter.