Fork anchored region from @ni/fast-foundation

[m-akinc]

Pull Request

🤨 Rationale

To better enable a fix for #3002, we decided to add an API to explicitly activate/deactivate anchored regions. Rather than adding new functionality to the @ni/fast-foundation package, we want to fork the implementation into nimble-components first, then add the new API there.

👩‍💻 Implementation

• Forked main impl file (index.ts), intersection-service.ts, and (meager) unit tests
  • Significantly refactored original updateLayout() implementation into smaller functions that use common logic for both vertical and horizontal updates, rather than separate functions with high levels of duplication.

Note to author: do not copy and paste LLM responses as PR descriptions. Do your own technical evaluations. If this PR is to be merged, this should be removed.

I had Sonnet 5 compare the refactor to the original, and it verified the correctness: >I compared `index.ts` against `anchored-region.ts` in the FAST foundation source (`anchored-region.ts`, the `@ni/fast-foundation` fork nimble depends on) method by method. Summary: the refactor is functionally faithful — I found no logic bugs. > >**What changed structurally** >The nimble version collapses FAST's duplicated horizontal/vertical code paths into shared per-axis helpers driven by a new internal `Dimension` type (`{ regionSize, anchorStart, anchorEnd, viewportStart, viewportEnd }`) ... >... >I traced every arithmetic expression (translation math for `start`/`insetStart`/`insetEnd`/`end`/`center`, viewport-lock corrections, available-space formulas, the "enough space vs. threshold vs. locktodefault" branching, and the direction-change reset check) and each one is mathematically identical to the original, just reparameterized. Notably: ... > >**Intentional (non-bug) deviations** >- Nimble uses the global `ResizeObserver`directly, while FAST casts `window` to a custom `WindowWithResizeObserver`/`ResizeObserverClassDefinition` (presumably for polyfill/SSR safety). This matches the convention used elsewhere in nimble-components (text-area/index.ts, tabs/index.ts, anchor-tabs/index.ts), so it's a deliberate, consistent simplification rather than a regression. >- `regionWidth`/`regionHeight` are typed as optional (`string | undefined`) in nimble vs. non-optional (but effectively unset initially) in FAST — a typing improvement, no behavior change. > >No functional deviations were found; the port preserves FAST's positioning algorithm exactly. - (incidental) moved `breadcrumb.foundation.spec.ts` from the root `breadcrumb` directory to the `tests` subdirectory

🧪 Testing

Automated tests pass.

[rajsite]

Talked to @m-akinc and emphasized that was looking towards re-implementing with popover api as part of the popover research task and want to avoid multiple back-to-back refactors of a shared low-level component if not necessary. Stated there wasn't an immediate rush so moving to draft.