[tiensonqin]

Summary

• Add opt-in Element.TrackLayoutWidth() and CommittedSpace.LayoutWidth() for non-scrolling boxes. Local-DIP client/content widths include padding, exclude borders, and preserve finite clipping constraints.
• Measure the box's own flow allocation and final direct-child inline-end border-box reach, including relative/absolute placement. Exclude hidden/leaving children, transforms and overflowing grandchildren; this is not recursive DOM scrollWidth.
• Reuse committed geometry lifetime and contentW, with a separate cache-input boolean. Existing scrolling metrics, dependencies and toolchain are unchanged. Document the downstream nowrap/nonshrinking-pill contract.

Conflict resolution: ordinary merge of main at fc67c1f293461d4db5814c06a88125e34e6c1355 into the original PR head 6fe0971afd238172731757343d6082e130744ce0, producing 420f520bffa776d0aa7838c0fafb1f8dac9795fd. The only textual conflict was two independent sections in docs/ui/migration.md; both are retained. Regenerated the UI facade and migration API index from the combined source. Navigation registration, its event-only cache classification, width tracking and its layout-cache input all remain. No feature redesign, dependency changes or rewritten history.

Testing

• Full headless CGO_ENABLED=0 go test -count=1 -timeout 20m -exec <owned-domain-wrapper> ./... passed. MYGO_E2E was absent in every child. Darwin tests run in the previously verified synthetic bundle, with a pre-main bundle-ID guard; isolated TestLocales also passed. The former baseline locale failure is fixed by fix: keep Darwin preferred languages global #25, now included from main.
• Focused race checks passed for the existing navigation, committed-width and layout-cache contracts (TestNavigationKeysRespectFocusedChildAndNearestOwner, TestCommittedLayoutWidth, TestLayoutInputs, TestLayoutCacheMatches, TestLayoutCacheReuses, TestLayoutCacheInvalidates). Race tooling uses CGO_ENABLED=1; production builds and ordinary tests remain cgo-free. No test assertions were changed for this merge.
• go generate ./ui and existing TestUIAPI pass; a second generation preserved the facade/index byte-for-byte. gofmt and both staged/unstaged whitespace checks passed.
• CGO_ENABLED=0 go build ./..., go vet ./..., and go vet -tags mygo_noinspector ./... passed for macOS/arm64, Linux/amd64+arm64 and Windows/amd64+arm64. JS/wasm source-package build/vet passed, excluding the CLI and test-only packages.
• bun install --frozen-lockfile, bun run build, bun run test (251 tests) and bun run typecheck passed.
• This conflict-only pass does not requalify the earlier full-suite race allocation-count discrepancy or Wasm/Node runtime failures. Neither was hidden, weakened or represented as resolved. No GUI/browser acceptance is claimed.

Release sizes against current main (fc67c1f2), identical Go 1.27.3, CGO_ENABLED=0, -buildvcs=false -trimpath -tags mygo_noinspector, production linker flags (-s -w, plus -H=windowsgui on Windows):

Platform	counter-native main → combined	hello delta
macOS arm64	9,782,050 → 9,782,082 (+32 bytes)	0
Linux amd64	9,961,735 → 9,961,735 (0)	0
Linux arm64	9,175,303 → 9,175,303 (0)	0
Windows amd64	10,585,088 → 10,586,624 (+1,536 bytes)	0
Windows arm64	9,570,304 → 9,571,840 (+1,536 bytes)	0

This is not fully size-neutral. Fresh unstripped -dumpdep/nm comparisons show computeBox → (*node).keepLayoutWidth (208 bytes on macOS ARM64; 384 bytes on Windows AMD64), plus growth of computeBox (32/64 bytes) and sameLayout (32/32 bytes), symbol/type metadata and final-file alignment. No new library/dependency or registry is introduced.

Tested on:

• macOS (headless unit/race checks and builds only)
• Windows (cross-build/vet and paired sizes only)
• Linux (cross-build/vet and paired sizes only)
• Web (wasm build/vet only)
• Other: no additional runtime validation in this conflict update

AI disclosure

• Tool and model: Devin, built by Cognition AI (underlying model identifier is not exposed).
• Used for: implementation, existing tests, conflict resolution, validation and this description.
• A human read every line of this change and can explain it.

Link to Devin session: https://app.devin.ai/sessions/79be43a0fc7b4ee3b783fa85566b6c1d
Open in Devin Desktop: https://app.devin.ai/desktop/session/79be43a0fc7b4ee3b783fa85566b6c1d?variant=devin
Requested by: @tiensonqin

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring