T-E078-layout-host-pane-shell

Status: closed/done · Impact: high · Complexity: large

Goal

Make the determined content area a layout-hosted pane group so panes persist across route changes and pages fill secondary/aux through the registry. Today each experience hand-rolls its own split inside its page (e.g. backup), so pane state resets on navigation and the registry has no host.

Today

Location	Role today
`packages/ts/shell/app/layouts/default.vue`	Renders `DAppShell` (sidebar/navbar/statusbar) with the routed page in the single `main` slot. No pane group.
`apps/determined/experiences/backup/app/pages/apps/backup/index.vue`	Hosts its own `PaneGroup` (secondary/main) inside the page — page-scoped, so it resets on navigation.

Proposed

DAppShell’s main slot hosts a persistent PaneGroup (secondary/main/aux) from @sksizer/panes, provided once by the layout so it survives navigation. DAppShell’s sidebar stays the app rail. main holds the routed page; secondary/aux are PaneOutlets fed by the registry (T-601N-pane-content-registry). Side panes render only when a page has registered content, so single-column experiences show just main. --pane-* maps onto --d-*. Backup drops its page-local PaneGroup and fills the regions via PaneContent.

Approach

In default.vue, wrap the routed page in a PaneGroup configured with secondary/main/aux; call providePaneRegistry(); map --pane-* onto --d-* on the container.
Render secondary/aux only when the registry holds content for that region, so unfilled experiences get no empty rails.
Migrate backup: remove its in-page PaneGroup; register the snapshot list as secondary and keep the detail in main, preserving selection and ?snap= deep-linking.
Confirm a single-column experience (e.g. search) still renders as one column with no empty panes.

Files to touch

Location	Kind	Change
`packages/ts/shell/app/layouts/default.vue`	modify	host `PaneGroup` + registry in the main slot
`apps/determined/experiences/backup/app/pages/apps/backup/index.vue`	modify	fill regions via the registry instead of a page-local `PaneGroup`

Acceptance criteria

AC-1: Backup’s snapshot-list/detail split renders via the layout-hosted pane group; snapshot selection and ?snap= deep-linking still work.
AC-2: Pane widths and collapsed state survive navigation within an experience (single layout-hosted instance).
AC-3: A single-column experience renders as one column, with no empty secondary/aux rails.
AC-4: secondary/aux inherit determined’s palette and dark mode via --pane-* to --d-*.

Out of scope

Keyboard/command wiring — T-IMOO-pane-toggle-keybindings.
Persistence across reloads — T-ARPY-persist-pane-state.
Sharing the shell with the SDLC dashboard (the generic-frame split) — a separate greenfield epic.

Depends on

T-601N-pane-content-registry

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