Here is a video of what I'm thinking about (be wary of your volume, the tab key is close to the mac's microphone!)
This only handles the Tab Key when editing a property, and thus is a part of DOMStorageDataGrid and not the generic DataGrid key handler.
Created attachment 33583 [details]
Tab Through DOM Storage DataGrid When Editing
- Relies on the (previously r+) patch in bug 27322