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This API allows the widget tree of an `OverlayPortal.overlayChild` to depend on various layout information (e.g. the incoming constraints, or the size of a RenderObject) from another child subtree of `Overlay` that does layout before the `overlayChild`. Most RenderObject subclasses can only access its child's or children's layout info, but not the layout info of its other descendants because of the "relayout boundary" optimization. Such locality makes the layout dependencies easier to reason about but it also makes it difficult to implement certain common UI patterns (see the examples in the description of the [previous PR](https://github.com/flutter/flutter/pull/163575)) The API is currently only available on `OverlayPortal` as it is the only Overlay API (AFAIK) that guarantees every render object in a "path" within the render tree has finished doing layout. TODO: polish the API docs and code comments TODO: more tests? TODO: markNeedsLayout should not imply markNeedsPaint in this case (or in layout builders in general). part1: https://github.com/flutter/flutter/pull/163575 diff this ... part1 https://github.com/LongCatIsLooong/flutter/compare/OverlayPortal-always-add-to-dirty-list...LongCatIsLooong:flutter:OverlayPortal-paint-transform-builder ## Pre-launch Checklist - [ ] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [ ] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [ ] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [ ] I signed the [CLA]. - [ ] I listed at least one issue that this PR fixes in the description above. - [ ] I updated/added relevant documentation (doc comments with `///`). - [ ] I added new tests to check the change I am making, or this PR is [test-exempt]. - [ ] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [ ] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord]. <!-- Links --> [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md