Record
Every record is one thing, and the page treats it that way. Three regions, stacked top to bottom. Identity — the name, the facts you check first (status · owner · version · dates), and the actions for this one thing; Data — everything it holds, in fields and sections; and Feed — the record's whole story, rendered by the Feed component: audit events and conversation woven into one stream, the way GitHub mixes system events and comments. Metadata lives in Identity (no properties rail) and audit lives in the Feed (no separate log) — so there's nothing on a side rail to break. That's why one simple vertical stack works at any width, split pane included.
Three regions, one stack — that's the whole layout. Identity sits at the top with the metadata, so there's no side rail to break when the pane gets narrow. Data and Feed scroll inside the well while the page stays put — the same scroll deal as a collection. Try Split pane: the stack just keeps working, because a stack has nothing horizontal to collapse. Audit and conversation share one Feed instead of two tabs — system events read as lines, replies as cards.