[devfive-relay-c70423]

Follow-up to #121, which merged at 55409c7 before this commit was pushed. Fixes Prime feedback 005. The audit is in reports/prime-feedback-2026-11.md → "005 and 009 resolved".

Problem

• A devup-ui investigation (main 675f7e20) found that Devup UI emits mix-blend-mode/filter/z-index/isolation/position/transform faithfully; its Chromium screenshots are byte-identical to plain CSS. The defect is in devup-mcp codegen.
• In the Prime SEO-GEO contact banner (instance 32:1929; decorations I32:1929;32:1876 and I32:1929;32:1877), the negative-z policy gave pos="relative" zIndex="0" to the transparent content wrapper inside the gradient owner.
• The multiply decorations (zIndex="-1") therefore composited against a transparent backdrop and drew white blobs: center pixel 224,224,224 instead of 77,44,208.

Change

• New codegen/layout/blend_backdrop.rs, called from push_layout_props. The stacking context moves to the nearest ancestor that paints the backdrop when:
  • a node's negative-z children are blended,
  • the node is a wrapper that paints nothing and opens no context of its own (no fill/stroke/effect/opacity/blend/rotation/own negative z),
  • and nothing in that ancestor is drawn before the wrapper.
• The wrapper keeps pos="relative" for the decoration's coordinates.
• These cases keep the existing containment on the wrapper:
  • unblended negative-z children,
  • wrappers that paint their own fill,
  • earlier painted siblings.
• The report records 005 as fixed and 009 as a Devup UI compiler bug fixed in @devup-ui/wasm 1.0.82 (devup-ui PR #681), with no MCP change.
• Adds a changepack (Patch).

Test evidence

• tests/blend_backdrop_context.rs: 4 regressions covering the lift onto the gradient owner (none on the wrapper) and the three fallbacks.
• Harness browser test (harness/runtime-audit/test/codegen-layout.test.mjs): compiles the fixture with the installed Devup UI and checks:
  • computed z-index is 0 on the owner and auto on the wrapper;
  • the decoration pixel is multiplied into the purple gradient;
  • a control that moves the context back to the wrapper shows the light blob, so the check tells the two cases apart.
• cargo fmt --all -- --check and cargo clippy --locked --workspace --all-targets --all-features -- -D warnings: pass.
• cargo insta test --workspace --all-features --check: 2264 passed / 1 failed / 2 ignored locally on Windows. No snapshot or golden changed.
  • The failure is r3_cancelled_client_recovers_retained_asset_job, a known load-dependent virtual-time test that fails only in the parallel full run (see fix: finish Prime feedback follow-ups (Korean wrap, heading margins, asset/theme/variant evidence) #121).
  • Run on its own, its composite_export binary passes 21/21.
• Node tests: 83 tests, 82 pass, 0 fail.
• Harness browser suite: 97 tests, 96 pass, 1 skipped (CI-only gate), 0 fail.

🤖 Generated with Claude Code

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