Note that default values only exist for optional arguments, so you actually need this instead:
    void scrollIntoView(optional (boolean or object) arg = true);

With that said, reading the spec there doesn't seem to be anything forbidding unions from having default values when used as optional operation arguments.

Is there any spec you're currently working on that uses those? Having a real life example always helps.

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Since there has been no response from the reporter, assuming there is no actual demand, I archive the issue.

IMHO, default values shouldn't be abused.  No use case looks good to me.