[pull] main from facebook:main#163
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…#34069) Found a couple of issues while integrating FragmentInstance#compareDocumentPosition into Fabric. 1. Basic checks of nested host instances were inaccurate. For example, checking the first child of the first child of the Fragment would not return CONTAINED_BY. 2. Then fixing that logic exposed issues with Portals. The DOM positioning relied on the assumption that the first and last top-level children were in the same order as the Fiber tree. I added additional checks against the parent's position in the DOM, and special cased a portaled Fragment by getting its DOM parent from the child instance, rather than taking the instance from the Fiber return. This should be accurate in more cases. Though its still a guess and I'm not sure yet I've covered every variation of this. Portals are hard to deal with and we may end up having to push more results towards IMPLEMENTATION_SPECIFIC if accuracy is an issue.
…pense boundary (#34201) This computes a min and max range for the whole suspense boundary even when selecting a single component so that each component in a boundary has a consistent range. The start of this range is the earliest start of I/O in that boundary or the end of the previous suspense boundary, whatever is earlier. If the end of the previous boundary would make the range large, then we cap it since it's likely that the other boundary was just an independent render. The end of the range is the latest end of I/O in that boundary. If this is smaller than the end of the previous boundary plus the 300ms throttle, then we extend the end. This visualizes what throttling could potentially do if the previous boundary committed right at its end. Ofc, it might not have committed exactly at that time in this render. So this is just showing a potential throttle that could happen. To see actual throttle, you look in the Performance Track. <img width="661" height="353" alt="Screenshot 2025-08-14 at 12 41 43 AM" src="https://github.com/user-attachments/assets/b0155e5e-a83f-400c-a6b9-5c38a9d8a34f" /> We could come up with some annotation to highlight that this is eligible to be throttled in this case. If the lines don't extend to the edge, then it's likely it was throttled.
Stacked on #34069 Same basic semantics as the react-dom for determining document position of a Fragment compared to a given node. It's simpler here because we don't have to deal with inserted nodes or portals. So we can skip a bunch of the validation logic. The logic for handling empty fragments is the same so I've split out `compareDocumentPositionForEmptyFragment` into a shared module. There doesn't seem to be a great place to put shared DOM logic between Fabric and DOM configs at the moment. There may be more of this coming as we add more and more DOM APIs to RN. For testing I've written Fantom tests internally which pass the basic cases on this build. The renderer we have configured for Fabric tests in the repo doesn't support the Element APIs we need like `compareDocumentPosition`.
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