⚠️ Deferred indefinitely (2026-03-12) — wgpu renderer migration suspended. Graphshell ships on egui_glow. See PLANNING_REGISTER.md §0.12.

Define a high-confidence gate for any future Glow → wgpu switch work rooted in WebRender evolution, while preserving current milestone velocity (UI/UX + workflow proof) on Glow.

This document is a readiness contract, not a runtime-switch authorization.

1. Glow remains active runtime composition policy for the current milestone.
2. WebRender/wgpu work starts now as readiness work (spikes, contracts, diagnostics parity, dependency wiring).
3. Renderer switch is blocked until all readiness gates in this document are closed with linked evidence.
4. Feature work continues, but must follow the guardrails below to prevent switch-cost inflation.

• A local patch path is demonstrated and documented for WebRender/Servo integration.
• Build inputs are pinned (fork branch/rev or Cargo patch) and reproducible in CI or scripted local setup.
• Rollback path back to Glow baseline is documented and tested.

• render_backend remains the sole ownership boundary for content-bridge mode selection.
• No direct new render_to_parent callback wiring is introduced outside backend-owned seams.
• Equivalent bridge-path/mode diagnostics are emitted for experimental WebRender/wgpu paths.

• No regressions in pass-order invariants (content pass before overlay affordance pass).
• No regressions in callback-state isolation guardrails and replay diagnostics.
• Non-interop capability fallback behavior is proven and observable.

• Target platform matrix for current milestone has explicit pass/fail state.
• Any platform-specific interop assumptions are documented with fallback behavior.
• Known gaps are tracked with issue links and tagged as switch blockers or non-blockers.

• Focused tests for bridge mode selection and fallback behavior are green.
• Replay snapshot schema remains stable (or change is documented with migration notes).
• No open stabilization bugs tied to render-path migration semantics.

All new feature slices touching rendering/composition must satisfy:

1. No new renderer-specific coupling in UI/workflow code

   • Feature code in workbench/UI modules must consume backend contracts, not backend internals.

2. Bridge metadata preservation

   • If a slice touches content-pass wiring, it must preserve bridge-path and bridge-mode observability.

3. Fallback-safe behavior

   • New behavior must define what happens when preferred rendering capability is unavailable.

4. Receipt-linked evidence

   • Every migration-adjacent feature slice posts tracker evidence proving guardrail compliance.

• Continue UI/UX and workflow-proof feature work on Glow.
• Require guardrail compliance for all render-adjacent slices.

• Run bounded spikes for local patching and integration seams.
• Keep switch logic policy-gated and disabled by default.
• Build evidence required by G1–G5.

Runtime switch from Glow baseline to WebRender/wgpu path is permitted only when:

• G1–G5 are all closed,
• closure evidence is linked in tracker,
• and a dedicated switch receipt confirms rollback validation.

Until then, Glow remains the production composition path.

• Backend migration tracker: #183.
• Related lanes: #88, #99, #92, #90.