cupertino backdrop filter / alert dialog wrong on web

[eseidelGoogle]

The dialog shows up with the discard button wrong. I think that's just a bug with backdrop filter, since the backdrop filter is also missing. @ferhatb fixed a bug a few days back, so possibly this is just a dupe of that: #54871

[yjbanov]

This is a duplicate of #49877.

[github-actions]

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