paseo/docs/expo-router.md

Expo Router

Paseo's mobile route tree is fragile because Expo Router and React Navigation do not fail loudly when a nested native route is mounted under the wrong layout. The usual symptom is a white or blank native screen with no JavaScript crash.

Read this before changing packages/app/src/app, startup routing, remembered workspace restore, or active workspace selection.

Ownership

Each layout owns only the routes directly inside its directory.

• The root layout registers h/[serverId].
• The root layout does not register host leaf routes such as h/[serverId]/workspace/[workspaceId], h/[serverId]/open-project, or h/[serverId]/index.
• packages/app/src/app/h/[serverId]/_layout.tsx owns the host leaves with relative screen names: index, workspace/[workspaceId]/index, agent/[agentId], sessions, open-project, and settings.

Expo Router warns with [Layout children]: No route named ... when a layout registers grandchildren. Treat that warning as a route-tree bug. On native, that shape can leave a nested index route mounted without its local dynamic params and render a blank screen.

Startup

The root / route chooses a host boundary. It does not jump directly into a host leaf.

• Good: / -> /h/[serverId]
• Bad: / -> /h/[serverId]/workspace/[workspaceId]

/h/[serverId] is the host home route. The host index restores the last remembered workspace for that host after the remembered selection has hydrated and the workspace has not been proven missing. If there is no restorable workspace, it goes to global /open-project.

This restore is based on the last navigated workspace, not current connection status. Do not redirect to another online host just because the remembered host is still connecting or offline; the workspace screen owns that offline/loading state.

This split is deliberate. The host layout must mount first so native local dynamic params exist before any nested workspace leaf is selected.

App-Wide Route Hops

When app-wide routes such as /new, /settings, or /sessions navigate back into a host workspace, use navigateToWorkspace(). Do not make the caller branch on its current route.

Pass only serverId and workspaceId for normal attention-aware navigation. When the action names a specific tab, pass it as target; that explicit choice is authoritative. Callers should not choose between separate route and tab navigation APIs.

The root stack owns h/[serverId]; the host stack owns workspace/[workspaceId]/index. Repeated global-route hops must POP_TO the root host route and pass the nested workspace screen when a host route is already mounted, or Expo Router can append extra hidden workspace deck entries. The workspace navigation helper inspects the mounted navigation state to make that decision; if no host route is mounted yet, it falls back to ordinary route navigation.

Those hidden entries are not harmless: composer floating panels can measure against the wrong deck and disappear offscreen.

Hidden host routes may keep their local params while an app-wide route is foregrounded. Active-workspace observers must prefer the current pathname and only use local param fallback during cold mount (/ or empty pathname), or a hidden workspace can overwrite the remembered workspace before Settings or History returns.

Params

Required dynamic params belong to the matched route.

Do not paper over missing required params by reading global params in the leaf. If useLocalSearchParams() misses a required param, fix layout ownership or the startup route shape.

Use the host route context for host-owned leaves that need the host id after h/[serverId]/_layout.tsx has matched. Do not make a leaf recover from an unmatched tree by guessing from global state.

App Directory

Keep non-route modules out of src/app. Expo Router treats ordinary .ts and .tsx files there as routes, which produces missing the required default export warnings and pollutes the route tree.

Put shared route policy in src/navigation, src/utils, stores, or another non-route directory.

Native Stack

Keep workspace identity and retention outside native-stack getId and dangerouslySingular. Expo Router maps dangerouslySingular to React Navigation getId, and getId has broken Android native-stack/Fabric by reordering an already-mounted workspace screen.

Use ThemedStack from packages/app/src/navigation/themed-stack.tsx for every Expo Router stack. React Navigation otherwise paints each native stack screen with its light default background. A screen-level wrapper can hide that surface while settled, but Android may expose it for one frame when navigation crosses from a nested stack to its parent stack. This is especially visible when an app-wide route such as /new opens from a dark workspace.

Do not read the active theme with useUnistyles() in a layout to build screenOptions. ThemedStack keeps that third-party prop theme-reactive through a small withUnistyles boundary without subscribing the route tree itself to every Unistyles runtime update.

Regression Shape

Pure helper tests are useful but not enough. The failure mode here is native route-tree state, so a real regression should launch native with seeded persisted state:

1. Seed paseo:last-workspace-route-selection with a valid { serverId, workspaceId }.
2. Launch the native app cold.
3. Assert a real screen is visible, not the blank tree.
4. Assert no [Layout children] warning appears.

The pure policy tests should still enforce the boundary split:

• root startup with a saved workspace returns /h/[serverId];
• host index with the same saved workspace returns /h/[serverId]/workspace/[workspaceId];
• host index with no restorable workspace returns /open-project.

Checklist

Before landing route changes:

• Did you change packages/app/src/app? Re-read this file.
• Did you touch remembered workspace restore? Keep root on /h/[serverId].
• Did a route return to a workspace? Use navigateToWorkspace() and pass a target when the action names a specific tab.
• Did you add a route? Register it in the layout that directly owns it.
• Did useLocalSearchParams() lose a required param? Fix the route tree.
• Did native show a blank screen without a crash? Suspect route ownership before stores, themes, or rendering.