Looks like we can't reproduce the issue or there's a problem in the test case provided.
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[GXT 3.0 PR] Grouped grid - can't toggle groups when using CellSelectionModel
GXT 3.0 Public Release
When CellSelectionModel is used as the selection model for a Grouped Grid, the expand and collapse functionality becomes in operative.
(CellSelectionModel appears to set the GridView.trackMouseOver variable to false [CellSelectionModel.java line 98], which prevents GridView from handling component events [GridView.java line 1746])