paseo/docs/mobile-panels.md

Mobile panels

Compact layouts have three mutually exclusive destinations:

• agent-list on the left
• agent in the center
• file-explorer on the right

They are one interaction, not two independent drawers. The implementation lives in packages/app/src/mobile-panels/.

Ownership

React/Zustand owns the durable intent:

interface MobilePanelSelection {
  target: "agent-list" | "agent" | "file-explorer";
  revision: number;
}

Every semantic target change increments revision. Repeating the current target is idempotent. Compact panel selection is not persisted; a cold start begins at agent.

The UI worklet owns transient motion:

• one normalized position (-1 left, 0 center, 1 right)
• the current motion target
• the active gesture's starting revision
• the last settled target

React also owns presentation lifecycle: whether an overlay is mounted/displayed and whether it may receive pointer events. Worklets never own display or pointerEvents.

Why one position

Both transforms and both backdrop opacities are derived from the same normalized position. Window width is only a projection input. Rotation changes the projection, not the panel state.

This makes these invalid states unrepresentable:

• a panel and its backdrop disagreeing
• left and right drawers both claiming to be open
• a width-sync effect resetting an active drag
• one animation context settling a transition owned by the other

Do not add another panel translate shared value, backdrop shared value, or width synchronization effect.

Ordering and interruption

A gesture captures the current revision when it becomes active. Per-frame updates are accepted only while that revision still owns the gesture.

When a React command arrives during a drag, its newer revision clears gesture ownership and starts motion toward the new target. The older gesture's remaining updates and finish callback are ignored. Canceled gestures return to the latest canonical target. Animation completion is accepted only when its target and revision still match the canonical command.

Manual gesture arbitration has two phases:

1. Before activation, determine whether horizontal intent may begin.
2. After activation, stop running begin checks and let the active revision own updates.

Re-running the begin gate after activation self-cancels the gesture because an active gesture is, by definition, no longer eligible to begin.

Integration rules

• Callers request semantic targets through panel-store; they never write shared values.
• Gesture behavior comes from the four explicit hooks in mobile-panels/gestures.ts.
• Keep SidebarModelProvider outside MobileGestureWrapper. The provider shares sidebar derivation across consumers, while Gesture Handler requires the wrapper's direct child to be a native View so its injected collapsable={false} reaches Android/Fabric.
• Mobile sidebars render through MobilePanelOverlay; do not duplicate overlay lifecycle or motion styles in sidebar components.
• The desktop left sidebar is retained too. App chrome owns separate mounted and visible decisions: closing it or yielding its width marks it inactive and applies display: none without conditionally removing the sidebar tree.
• Animated panel nodes use React Native static styles plus inline theme values. Do not attach Unistyles-generated styles to those nodes; Unistyles and Reanimated patching the same Fabric node has caused native crashes.
• The plain React wrapper owns display: none after settlement. This prevents a stale Fabric animated prop commit from resurrecting a closed overlay.
• Hidden tabs and workspaces use RetainedPanel. It owns a non-collapsible native root, visibility, pointer events, and the active signal consumed by useRetainedPanelActive.
• Panels whose gesture wrapper already owns visibility use RetainedPanelActivity to provide the same active signal without adding another layout root. Persistent animations, timers, polling, and shared clocks must subscribe to that signal and stop when their final visible consumer leaves.
• Synchronized step animations use one wall-clock-aligned source. Register a local shared value only while its retained panel is active so hidden animated styles remain mounted without receiving clock updates. Do not give every instance its own loop or leave hidden styles subscribed to the source.
• Retention order and render order are separate concerns. LRU metadata may change on every switch; keyed retained roots must keep a stable sibling order. Moving large retained roots triggered Fabric Differ failures (addViewAt / removeViewAt view reuse) on Android.
• The newly active panel must be included in the same render that changes selection. Adding it from an effect creates a committed frame where every retained panel is hidden, which is a real blank screen.
• Do not suspend retained native subtrees with Suspense/react-freeze. Suspension changes native ownership and can detach descendants. Keep the tree mounted, stabilize its subscriptions/selectors, and use the retained-panel active signal to stop timers, polling, and other genuine background work.

Tests

packages/app/src/mobile-panels/model.test.ts exercises command, drag, cancellation, interruption, rapid-command, stale-completion, and width-projection sequences through the transition model. Add a sequence there whenever ownership or ordering changes.