Can share some more details - how do you use the mentioned ItemsSource? However I assume you might need to reach the DataContext of the TextBlock. The sender in the MouseLeftButtonDown event handler there would be the TextBlock, so you might need to check in the TextBlock DataContext as shown below:
sender, MouseButtonEventArgs e)
var txtBlock = sender
var vm = txtBlock.DataContext
// this is the ViewModel class name
// PlannerTagSerializable ob = (PlannerTagSerializable)sender;
Hope this helps.
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