Description

When using MCP apps that request a fullscreen display mode (via ui/request-display-mode), the desktop client
does not handle the request — the app remains constrained to the inline iframe within the chat view.

This is particularly problematic for apps like drawing tools or dashboards where the inline view is too small
to be usable.

Steps to Reproduce

1. Add the Excalidraw MCP app as an extension: https://excalidraw-mcp-app.vercel.app/mcp
2. Interact with the app so it renders inline in the chat
3. The app attempts to request fullscreen display mode via ui/request-display-mode
4. Nothing happens — the app stays in the small inline iframe

Expected Behavior

The desktop client should handle ui/request-display-mode with mode: "fullscreen" by opening the MCP app in a
dedicated, resizable window, as defined in the MCP ext-apps specification (SEP-1865)
(https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/draft/apps.mdx).

Actual Behavior

The ui/request-display-mode method is not implemented. The app is stuck in the inline iframe with no way to
expand, making canvas-based apps like Excalidraw effectively unusable.

Environment

• Goose Desktop (Electron)
• Excalidraw MCP App: https://excalidraw-mcp-app.vercel.app/mcp