Enable iframe sandbox relaxation (allow-downloads) for Fabric data apps

Problem statement

Summary

File downloads are impossible from a Rayfin app running inside the Fabric Portal, because the Fabric data app workload renders the app in a sandboxed iframe without the allow-downloads attribute. It would be great if Rayfin enabled Fabric's sandbox relaxation mechanism for data apps, so that apps generating files (documents, exports, reports) can offer downloads directly.

Current behavior

• The app is deployed with rayfin up and runs inside the Fabric Portal in an iframe sandboxed with allow-same-origin allow-scripts only.
• Any download attempt from inside the iframe — <a download>, file-saver's saveAs(), window.open() with a blob URL — is silently blocked by the browser. This is intended browser behavior for sandboxed iframes without allow-downloads (see Chrome Platform Status and the WHATWG discussion), so there is no client-side workaround.
• As a workaround, we currently show a notice asking the user to copy the app URL and reopen it in a regular browser tab, where downloads work. This is a poor experience for an app whose core output is a downloadable file.

Expected behavior

Downloads initiated by user action inside the app should work while the app is embedded in the Fabric Portal.

Environment

• Rayfin guide/docs version: 1.33.2
• Deployment target: Fabric data app (managed workload, rayfin up)
• Browsers tested: Chromium-based (behavior is per HTML spec, so all modern browsers are affected)

Use case

Our app converts user-uploaded .docx files and produces new documents. The download of the generated file is the app's primary output, so the current limitation forces every user through the "copy link → open in a new tab" detour on every use.

Proposed solution

Proposed solution

Fabric already has an official mechanism for this: workloads built with the Extensibility Toolkit can set <EnableSandboxRelaxation>true</EnableSandboxRelaxation> in the workload manifest. After an additional user consent, the iframe is granted allow-downloads, allow-forms, and allow-popups — see How-To: Relax the iFrame.

Since the Fabric data app workload manifest is managed by Rayfin, app developers cannot opt into this themselves. Could Rayfin:

1. Enable sandbox relaxation on the Fabric data app workload (with the standard consent flow), or
2. Expose it as an opt-in setting (e.g., in rayfin.yml) for apps that need downloads?

Alternatives considered

No response

Additional context

No response