The reproduction steps are probably not exhaustive, the issue does not reproduce with the nominal case. I managed to reproduce locally by playing with several tabs:
Install 17.10.12 and complete the wizard with the XWiki Standard Flavor.
Edit XWiki.AttachmentSelector and change the path:${action.url} line, so the upgrade has a merge conflict there.
Restart on 17.10.13, open the Distribution Wizard, start the flavor upgrade and stop at the merge conflict question.
Open the wizard in a second tab and answer the questions there, until the flavor is installed.
The first tab still shows the question, with Skip/Cancel and no Continue. F5 shows Continue.
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