Send execution requests from frontend in progressive rendering mode

[trungleduc]

References

• This PR refactors progressive rendering in Voilà, enabling the frontend to dispatch cell execution requests incrementally via the Jupyter kernel websocket API instead of a custom execution request endpoint

• Notebook cell execution is now fully compatible with the Jupyter kernel API, reducing reliance on Voilà-specific server endpoints and the nbconvert execution mechanism. As a by-product, Rendering stuck randomly #1428 is fixed in the new progressive rendering mode

vl22.mp4

Code changes

Frontend

• The frontend now loops through notebook cells and sends execution requests one at a time using the kernel websocket protocol (instead of a custom execution request handler).
• Output areas for each cell are created and attached dynamically; visual skeletons are removed as soon as execution completes.
• To avoid leaking notebook source code, the execution code is the cell index, which the server translates to the actual cell source before forwarding the message to the kernel.
• The rendering logic is simplified: outputs are updated in-place, and loading indicators are managed per cell.

Backend

• The custom WebSocket-based ExecutionRequestHandler is removed. Instead, progressive rendering is now handled via the standard kernel websocket handler (VoilaKernelWebsocketHandler).
• The handler intercepts execute_request messages, replaces the code payload (which is the cell index) with the actual notebook cell source, and passes the message to the Jupyter kernel.
• Output and error messages are masked as needed based on configuration.

User-facing changes

N/A

Backwards-incompatible changes

• The custom execution endpoint /voila/execution/ is removed
• The kernel handler endpoints /api/kernels/ and /api/kernels/%s/channels are scoped under the /voila/ prefix and become /voila/api/kernels and /voila/kernels/%s/channels

[github-actions]

👈 Launch a Binder on branch trungleduc/voila/new-exec

[trungleduc]

Due to the breaking change, this feature should be released in 0.6