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Document MCP Apps resize handling improvements #1075

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Document MCP Apps resize handling improvements #1075

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Update docs/contributing/mcp-apps-architecture.mdx
mintlify[bot] Dec 12, 2025
5ca02b8
Update docs/contributing/mcp-apps-architecture.mdx
mintlify[bot] Dec 12, 2025
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16 changes: 16 additions & 0 deletions docs/contributing/mcp-apps-architecture.mdx
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Expand Up @@ -123,12 +123,18 @@ sequenceDiagram
Located in `client/src/components/ui/sandboxed-iframe.tsx`:

```typescript
export interface SandboxedIframeHandle {
postMessage: (data: unknown) => void;
getIframeElement: () => HTMLIFrameElement | null;
}

export const SandboxedIframe = forwardRef<SandboxedIframeHandle, SandboxedIframeProps>(
function SandboxedIframe({ html, sandbox, onMessage, ... }, ref) {
// Outer iframe loads sandbox proxy
// Waits for ui/notifications/sandbox-ready
// Sends HTML via ui/notifications/sandbox-resource-ready
// Forwards all non-sandbox messages to parent handler
// Exposes getIframeElement() for direct DOM access (e.g., resizing)
}
);
```
Expand Down Expand Up @@ -205,6 +211,16 @@ window.mcpApp = {
};
```

### Resize handling

When a widget calls `resize()` or sends `ui/notifications/size-changed`, the host:

1. Ignores resize requests in fullscreen/PiP modes (they use 100% height)
2. Applies border-box compensation to prevent feedback loops
3. Animates the iframe height change using the Web Animations API
4. Directly manipulates the iframe element (not React state) for performance
```

### Events

Widgets can listen for these events:
Expand Down