[BUG] Detached/new thread window is missing features: IME double-input, no sidebar, plan panel not shown (macOS desktop app)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Summary

When a thread is opened in a NEW window (detached from the main window) in the
Claude desktop app, the new window behaves as a stripped-down view and is missing
several features that work correctly in the main window.

Note: all three problems below occur ONLY in the detached/new window. The main
window works correctly. This is a window feature-parity bug, not the previously
reported CLI/terminal IME issues.

Environment

• Claude desktop app: 1.9659.4
• Claude Code (CLI): 2.1.159
• macOS: 26.0 (build 25A8353), Apple Silicon (arm64)
• Input method: Google Japanese Input (Google日本語入力 / Mozc), NOT the default macOS IME
• Running inside the Claude desktop app (GUI), not a terminal (the bug is about the app's detached window).

Observed behavior (3 issues)

1. IME double-input: Typing a single key produces two characters

(e.g. pressing "a" yields "aあ"). The raw key and the composed/confirmed
character both appear. Reproduced with Google Japanese Input (Mozc).
The SAME IME works correctly in the main window, so this is window-specific,
not a general Google-IME problem.

1. No left sidebar: The new window does not show the threads/menu sidebar

that is present in the main window.

1. Plan mode panel missing: After the assistant produces a plan in plan mode,

clicking "Open plan" does nothing in the new window. In the MAIN window, the
same action correctly opens the plan in a right-side panel.

Related issues (not duplicates)

• #22853 (closed) and #8502 / #1306 / #12332 / #14814 cover IME problems in the

terminal/CLI (React Ink). This report is different: the double-input happens
ONLY in the detached desktop window — the main window handles the Japanese IME
correctly. This points to a window-parity defect, not the general CLI IME issue.

• #50856 / #60003 / #25524 / #58627 concern the main window's sidebar failing

to populate. This report is the opposite: a NEW window that lacks the sidebar
entirely while the main window's sidebar works.

• #52743 (multi-window / --desktop targeting) is related context for new-window support.

Notes

• The main window (the one shown at app launch) works correctly for all three.
• The IME issue is reproducible with Google Japanese Input (Mozc).

What Should Happen?

Expected behavior

A thread opened in a new window should have full feature parity with the main
window: correct IME handling (no double-input), the left sidebar, and a working
plan panel.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce

1. Launch the Claude desktop app (main window with the left sidebar).
2. Open a thread in a new/separate window.
3. Try to type in the input box (with Google Japanese Input), and try plan mode.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.159

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_