Milimo Frontend State Manager Skill

As the Milimo Frontend State Manager, your primary responsibility is maintaining the integrity, performance, and synchronization of the React 18 UI.

1. The Zustand "God Store" (timelineStore.ts)

Milimo uses a single timelineStore.ts built from 7 distinct slices to prevent context re-render thrashing:

• projectSlice (Project metadata, save/load)
• shotSlice (Storyboard elements, generation status)
• trackSlice (Timeline lanes, clip positioning)
• playbackSlice (currentTime, playing status, maximum duration cache)
• uiSlice (Tooltips, panel visibility)
• elementSlice (Project elements gallery)
• serverSlice (SSE handling, active jobs dictionary)

RULES:

• No independent React Contexts for global state.
• Always use the <Toggle> component for switches.
• Components subscribing to frequent updates (like currentTime from PlaybackEngine) MUST use selective useStore(useShallow(state => ...)) bounds or access .getState() directly via useRef to avoid React reconciliation loops.

2. Magnetic V1 & Free Placement (Timeline Layout)

The timeline UI is entirely DOM/CSS-based absolute positioning (no HTML5 <canvas>).

• Centralized in computeTimelineLayout() inside timelineUtils.ts.
• Track 0 (V1) behaves magically: It ignores user-specified startFrame. It calculates layout by iteratively appending the duration of Shot N-1 to Shot N. Dragging a clip on V1 simply reorders the array.
• Track 1 (V2) & Track 2 (A1) are "free placement": Clips are absolutely positioned based on their explicitly defined startFrame, allowing overlapping overlays and audio.

3. Browser Quirks & Engine Workarounds

Safari is notoriously difficult for media web apps. The frontend must strictly adhere to these workarounds:

A. Safari Audio Fix (GlobalAudioManager.ts)

Safari blocks <audio> autoplay and suffers from aggressive media desync when syncing HTML5 video to separate audio tags.

1. The Playback Loop: PlaybackEngine.tsx uses a headless requestAnimationFrame to advance currentTime via Date.now() deltas.
2. Audio Decoding: GlobalAudioManager.ts pre-fetches .mp3 files via HTTP Range requests and uses decodeAudioData into an in-memory AudioBufferSourceNode.
3. Mute Control: Audio nodes remain playing but are muted (gain.value = 0) instead of being stopped, circumventing Safari user-gesture policies on re-start.
4. Drift Tolerance: The frontend hardcodes a 250ms drift tolerance. If the video <video> tag current time deviates from the PlaybackEngine time by more than 250ms, it forces a .fastSeek().

B. Safari Draggable Hover Bug

Elements with draggable={true} in Safari WebKit silently swallow CSS pointer events, completely neutralizing Tailwind's group-hover and standard :hover pseudo-classes intended for child overlay UI.

• DO NOT use CSS hover states for interactive buttons inside draggable elements.
• INSTEAD, use explicit React state (e.g., const [isHovered, setIsHovered] = useState(false)) bound to onMouseEnter and onMouseLeave synthetic events (which Safari does reliably fire) to toggle UI visibility.

4. SSE & React Hooks

Backend task updates flow exclusively through Server-Sent Events.

• SSEProvider.tsx manages a single robust EventSource with exponential backoff.
• The Zustand serverSlice.ts exposes handleServerEvent() which patches the shotSlice implicitly on "progress", "cancelled", and "complete" messages.
• Job Poller: SSE dropouts happen. On initial page mount, jobPoller.ts grabs all shots marked isGenerating: true and fires explicit GET /status/{job_id} calls to hydrate the UI.

5. UI Layout Components

• The layout revolves around Layout.tsx, wrapping a top toolbar, CinematicPlayer.tsx center stage, VisualTimeline.tsx sticky at the bottom, and a right-side inspector (InspectorPanel.tsx, MediaLibrary.tsx, StoryboardView.tsx mutually exclusive visible tabs).
• Drag-and-drop between Library and the Inspector is supported via HTML5 drag APIs.