Allow /tui screen mode in background sessions on Windows (agent-view workflow has no garbling workaround)

Summary

On Windows Terminal, /tui screen (a.k.a. /tui fullscreen) eliminates the well-known left-edge character garbling that occurs during scrollback. However, /tui is gated to foreground sessions only, which means any session reached via agent view cannot apply the workaround. For users whose primary workflow is agent-view-centric (multi-tab launcher pattern: claude agents hub + dispatched/detached sessions), there is currently no working mitigation.

Symptom

In Windows Terminal, the first 1-2 character columns on the left edge of the viewport show stale/garbled fragments when scrolling. Reproducible on Windows 11 + Windows Terminal stable + Claude Code 2.1.141 in both cmd- and pwsh-spawned panes. Switching the spawn shell does not affect the renderer.

Current state

/tui screen resolves the garbling cleanly in foreground sessions. /tui default restores prior behavior. Tested working.

Attempting /tui screen (or /tui fullscreen) in a background session returns:

Renderer switching isn't available in a background session — press ← to detach and run /tui from a foreground session.

"Background session" includes:

1. Sessions opened via claude agents (the agent view itself)
2. Sessions dispatched from agent view
3. Foreground sessions that were detached back to agent view (pressing ← on an empty prompt)

For a launcher that opens claude agents as the primary hub in each tab and dispatches work from there, items (1)-(3) cover effectively all panes a user interacts with for the bulk of a working day.

CLAUDE_CODE_NO_FLICKER=1 was tried previously and does not resolve the garbling on this setup.

Ask

Either:

(a) Allow /tui screen (alternate-screen renderer) inside background sessions. The underlying renderer constraints presumably differ from foreground sessions, but on Windows the alt-screen mode is what unblocks Windows Terminal scrollback rendering, and the workflow cost of not having it is high.

(b) Provide an opt-in mechanism — env var (e.g. CLAUDE_CODE_TUI_MODE=screen) or ~/.claude/settings.json key — that defaults background and agent-view sessions to screen-mode rendering at startup, bypassing the runtime switch.

Either path unblocks Windows users whose primary workflow runs through agent view.

Environment

• OS: Windows 11 Pro 26200
• Terminal: Windows Terminal (stable)
• Claude Code: 2.1.141
• Shell tested: both cmd and pwsh (no difference)

Related

• #19637 — canonical Windows Terminal rendering thread (underlying render bug)
• #12335 — Windows flickering tracker
• #769 — umbrella TUI complaint thread

This issue is distinct from those: it specifically asks for the existing /tui screen workaround to be reachable in the background-session contexts where it would help most.