Looks like we can't reproduce the issue or there's a problem in the test case provided.
Filter grid header shows no indication of active filter
Version(s) of Ext GWT
Ext GWT 3.0 Release Candidate 1
Browser versions and OS(and desktop environment, if applicable)
Firefox 8, IE 9 Windows 7
When a grid filter is applied, there is no visual indicator in the grid header to show a filter is active. In previous releases the column header text would be italicized to show that the filter is active. A user can still open up the column menu and see the filter box is checked, but this requires an extra click per column in the grid to see which columns are actively being filtered.
Remote Filter Grid example: http://staging.sencha.com:8080/examp...motefiltergrid
Header filtered style
When a column has a filter applied to it, the css class of the header is changed to x-filtered-column. Adding styles to this class with will change the header when the column is filtered.