Looks good for me & working as expected with another similar use case (VLSuite modal), RTBC +1.
I'm not sure this happens if it opens in the sidebar too, but it does happen when you alter the link to open in a popup instead.
Patch incoming, it perfectly copies core's approach to closing dialogs, which seems to work with sidebar as well as popup style.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Looks good for me & working as expected with another similar use case (VLSuite modal), RTBC +1.