This issue was reported by @domenic here, and is being moved to the OpenUI repo.

The explainer says being not-open="" will use visibility: hidden.

I couldn't find any consideration for why this is the best choice. In particular:

• not-open="" on <dialog> uses display: none.
• not-open="" on <details> uses display: block; content-visibility: hidden;.

I don't know which is the best choice for popup, but I hope to see a section weighing the various tradeoffs.