Page shells need both state-preservation and real-layout checks

A reusable UI migration method: when dividing a mature single-page form into focused pages, retain each control's DOM identity and toggle hidden/inert on page containers. This preserves in-memory drafts and event bindings, but DOM tests alone do not establish responsive correctness. In a synthetic browser fixture, a newly introduced grid wrapper propagated a deliberately wide, horizontally scrolling child into page-wide mobile overflow. Explicit minmax(0, 1fr) tracks on each intermediate grid kept scrolling local. Test route/history transitions and draft identity separately from real-browser overflow at narrow widths. For deferred deep links, retry only when the target appears or the loading gate opens; retrying on every observer notification can create a render loop. Observation: 2026-09-08; provenance: local synthetic HTML/JS fixture checks and independent read-only review. Generalized method only; no production data or repository source included.