feat(wasm): render notebook snapshot while Pyodide initializes

[mscolnick]

Previously the WASM loader blocked the entire app behind a spinner until
Pyodide finished downloading. With the new snapshot embedded by
marimo export html-wasm --execute, render cells (code + cached outputs)
on first paint and let Pyodide download in the background.

• Gate spinner on whether cells exist, not whether Pyodide is pending
• Toast Pyodide init failures instead of throwing when a snapshot is shown
• Add a footer "Pyodide" indicator that mirrors the "Kernel" one

[vercel]

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 11, 2026 6:53pm

[cubic-dev-ai]

No issues found across 7 files

Architecture diagram

sequenceDiagram
    participant UI as React UI
    participant Footer as Footer Component
    participant PyStatus as PyodideStatus (NEW)
    participant PyApp as RunApp (CHANGED)
    participant PyLoader as PyodideLoader (CHANGED)
    participant Cells as Cells Store
    participant Bridge as PyodideBridge
    participant Pyodide as Pyodide Runtime

    Note over UI,Pyodide: Start-up Sequence (snapshot path)

    UI->>PyApp: Render notebook
    
    alt Snapshot with cells present
        PyApp->>Cells: hasCellsAtom
        Cells-->>PyApp: true
        Note over PyApp: Skip spinner gate — show cells immediately
        PyApp->>UI: Paint cells (code + cached outputs)
    else No cells yet
        PyApp->>Cells: hasCellsAtom
        Cells-->>PyApp: false
        PyApp->>UI: Show spinner (original behavior)
    end

    Note over UI,Pyodide: Background Pyodide Initialization

    UI->>PyLoader: Mount (parallel with cell render)
    PyLoader->>Cells: hasCellsAtom
    PyLoader->>Cells: hasAnyOutputAtom
    PyLoader-->>PyLoader: shouldShowSpinner(hasCells, hasOutput, mode, codeHidden)

    alt shouldShowSpinner = true (nothing visible)
        PyLoader->>UI: Show WasmSpinner (full-screen)
    else hasCells and (code visible or has output)
        PyLoader->>UI: Render children (snapshot stays visible)
    end

    PyLoader->>Bridge: Wait for initialized.promise
    Bridge->>Pyodide: Initialize (download + boot)

    alt Pyodide ready
        Bridge->>Bridge: set wasmInitStatusAtom = "ready"
        Bridge-->>PyLoader: Promise resolves
        PyLoader->>PyLoader: No longer pending
    else Pyodide fails
        Bridge->>Bridge: set wasmInitStatusAtom = "error"
        Bridge-->>PyLoader: Promise rejects

        alt Snapshot is showing (nothingToShow = false)
            PyLoader->>UI: toast("Failed to start notebook runtime")
            Note over PyLoader: Error is surfaced as toast, snapshot remains visible
        else No snapshot (nothingToShow = true)
            PyLoader->>UI: throw error (caught by error boundary)
        end
    end

    Note over UI,Pyodide: Footer Indicator (NEW)

    Footer->>PyStatus: Render component
    PyStatus->>PyStatus: Check isWasm() and wasmInitStatusAtom

    alt Status = "ready" or not WASM
        PyStatus->>PyStatus: Return null (hidden)
    else Status = "loading"
        PyStatus->>UI: Show spinner + "Pyodide" label
    else Status = "error"
        PyStatus->>UI: Show error icon + "Pyodide" label
        PyStatus->>PyStatus: Tooltip: "Pyodide failed to initialize"
    end

    Note over Footer: Mirrors Kernel status indicator behavior

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[Copilot]

Pull request overview

Enables WASM (Pyodide) notebooks exported with an embedded snapshot to render immediately (cells + cached outputs) while Pyodide continues initializing in the background, instead of blocking the entire UI behind a loader.

Changes:

• Update the WASM loader gating logic to show a spinner only when there’s nothing meaningful to render yet (no hydrated cells, or headless run mode with no outputs).
• Surface Pyodide init failures via a toast (when snapshot content is already visible) and add a footer “Pyodide” status indicator.
• Skip the “Connecting…” gate in run mode when cells already exist (e.g., from an embedded snapshot).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
frontend/src/core/wasm/state.ts	Adds a wasmInitStatusAtom to track Pyodide init state.
frontend/src/core/wasm/PyodideLoader.tsx	Stops blocking render on Pyodide init; introduces shouldShowSpinner predicate and error-toasting behavior.
frontend/src/core/wasm/bridge.ts	Updates WASM bridge to set init status to ready/error based on worker messages.
frontend/src/core/wasm/tests/PyodideLoader.test.ts	Adds unit tests for the spinner gating predicate.
frontend/src/core/run-app.tsx	Avoids showing the connecting spinner when cells already exist.
frontend/src/components/editor/chrome/wrapper/footer.tsx	Adds the Pyodide status indicator to the footer.
frontend/src/components/editor/chrome/wrapper/footer-items/pyodide-status.tsx	Implements the footer Pyodide init indicator and tooltip.

[dmadisetti]

+1 Nice

[github-actions]

🚀 Development release published. You may be able to view the changes at https://marimo.app?v=0.23.6-dev32