conditional

Past conditional: If the door had been open, it would have attracted Harry's attention.
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The door was open, and it did attract Harry's attention.

It is a conditional in the past that was actually realized.

AlpheccaStarsIf the door had been open

Thank you for your answering. But "If the door had been open" is factual, not counterfactual according to the previous context.

I don't think it's a conditional. I think it's the use of would for an assertion of certainty. Dumbledore is certain that the open door was bound to attract Harry's attention.

fivejedjonI don't think it's a conditional. I think it's the use of would for an assertion of certainty. Dumbledore is certain that the open door was bound to attract Harry's attention.

Then I suppose you prefer the following paraphrases of the non-past and the past time versions.

Naturally, it would attract your attention. ~ Naturally, it is bound

CalifJimNaturally, it would attract your attention. ~ It's easy to imagine it attracting your attention.Naturally, it would have attracted your attention. ~ It's easy to imagine it having attracted your attention.

They are fine. I think would expresses a stronger certainty that it's easy to imagine, but that's a personal feeling.

contraposition"If the door had been open" is factual,

That is not a factual statement. It is a conditional, because it begins with "if"

eg.
If it is raining, you can borrow an umbrella.
Does that statement tell you that it is raining or not?

A conditional is not always counterfactual:

A: My umbrella is gone!
B: John must