Feature request: native sub-issue hierarchy in Copilot issue viewer (and extension hook support)

[rianfowler]

Summary

We evaluated sub-issue visibility in the Copilot app issue workflow and found there is currently no documented way to customize the built-in issue viewer to show parent/sub-issue hierarchy.

Request

Please add support for at least one of these:

1. Native sub-issue hierarchy display in the built-in Copilot issue viewer (parent + ordered immediate sub-issues).
2. An extension hook/API surface that lets extensions augment the issue viewer with sub-issue sections.

Why this matters

Teams using GitHub sub-issues need hierarchy context while working inside Copilot issue sessions. Without native support, teams must build and maintain separate custom canvases for a common planning workflow.

Current workaround and findings

• We built a dedicated canvas extension (sub-issues-viewer) with GitHub GraphQL + HTMX.
• HTMX worked well for fast iteration and lightweight interactivity.
• The workaround is feasible, but still requires extra UX/design work and duplicates context that already exists in the built-in issue viewer.

Suggested MVP

• In issue sessions, show: parent summary + ordered immediate sub-issues list.
• Provide quick actions: open child issue in GitHub / open in Copilot issue session.
• Respect existing issue permissions and repository boundaries.

Extra question

If native support is not planned soon, is there a recommended/official extension pattern for issue-viewer augmentation so teams can build this without relying on undocumented host internals?

Thanks for considering this request.

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