This issue duplicates another issue.
Thanks for the report, we've already got this one on the radar as: EXTJSIV-7319
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Focus is lost when you type a value and it is equal to one of element in expanded list
if you have time format "H:i", then if you type 11:56 (but increment is 10 minutes), value will be valid and focus won't be lost. But if you type 11:50 focus will be lost...
This ticket has been targeted for 4.2