Noun clause or adverb clause? [closed]

I was reading about noun clauses and adverb clauses and I am a little confused about what follows:

One grammar claims that the clause in bold in the following sentence is a noun clause working as an adjective complement:

I’m happy that you’ve decided to come.

Another grammar says that the sentence in bold in the following sentence is an adverb clause:

Many English speakers today are surprised that Early English would be unintelligible to them.

I am confused, because both clauses follow an adjective and, because they start with “that,” I think they are, indeed, noun clauses. Moreover, the second clause doesn’t seem to work as an adverb, once it doesn’t answer “when?”, “how?”, “why?”, “where?”, etc.

Can anyone help me, please?

[1] I’m happy [that you’ve decided to come].

[2] Many English speakers today are surprised [that Early English would be unintelligible to them].

I would strongly recommend dropping the terms 'noun clause', 'adverb clause' etc. The classification of finite subordinate clauses is based on their internal form rather than spurious analogies with the parts of speech.

In both your examples, the bracketed expressions are declarative content clauses functioning as complement of the adjectives "happy" and "surprised".