Looks like we can't reproduce the issue or there's a problem in the test case provided.
Have to manually include css and image for xtype: 'checkcolumn'
CSS problem still exists. By just setting xtype: 'checkcolumn', you don't get checkboxes in your grid...It's not obvious that you have to manually include the CSS for xtype: 'checkcolumn'
The checkcolumn isn't part of the core library, so it's expected that there may be other resources that are not part of the core css.
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Thanks for pointing out the need to include the CSS for the checkboxes. I was wondering why I couldn't see anything... "non obvious" is an understatement :-)