Fullscreen Host Mode
Host-managed fullscreen mode for deeper work, plus the interior workspace payload that belongs inside that expanded host canvas.
Fullscreen Host Mode
Fullscreen is a host-managed expanded mode requested through the Apps SDK. The registry contract here only covers the interior workspace payload rendered inside that host canvas.
What it is for
Document fullscreen as host behavior first, then show the workspace payload that fits inside that host-owned expanded canvas.
Use when
The task needs more space than an inline card, rail, or PiP can safely provide.
Do not use when
You are treating fullscreen like permission to replace chat with a standalone app shell.
Related patterns
App Surfaces · Fullscreen Workspace · MCP Client Interface · Widgets & Registry · Inspector Rail
Escalation path
Inline or rail -> request host fullscreen for deep work -> use the host composer for follow-up prompts.
What This Page Should Mean
This is not the app taking over the screen. It is the host granting more room while still keeping the conversation and composer model intact.
What The Registry Owns
The registry owns the interior workspace payload: the task canvas, supporting context, and action strip that live inside the host-managed fullscreen mode.
Mock ChatGPT fullscreen viewport
The host expands the task canvas, but the composer still exists below the app response area.
Interior workspace payload
Estimate review workspace
Selection summary
Fullscreen should deepen engagement, not recreate a standalone app header, sidebar, and composer.