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The current way to search projects provides searching via dropdown, but it still reacts to enter by submitting the form (and not doing any search then). This is quite unintuitive, because a lot of people are used to just input the search term and press enter, before any dropdown appears (and therefore the user doesn't know there's supposed to be a dropdown). I'd suggest either:
Fixed in https://pagure.io/pagure/pull-request/826 by using the second approach you listed (suppress the submission with the enter key)
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