[BUG] Sub-agent messages render in wrong vertical order — insertion position resets on main↔sub-agent context switch

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?

When a sub-agent makes multiple sequential tool calls (each in a separate assistant message), the messages render in the wrong vertical order in the inline sub-agent expansion. Some messages appear above the initial prompt, while others appear below it.

It appears that whenever the TUI switches context between the main conversation and the sub-agent panel, the output insertion position resets to the top of the sub-agent's output region, causing new content to be prepended rather than appended chronologically.

The bug reproduces in both fullscreen (/tui fullscreen, using mouse click to switch) and classic/default TUI modes (using keyboard navigation to switch).

What Should Happen?

Sub-agent messages should render in chronological order, top-to-bottom:

1. Initial prompt (❯)
2. First tool call + result
3. First summary
4. Second tool call + result
5. Second summary
6. ...

Steps to Reproduce

1. Launch Claude Code (either fullscreen or classic TUI mode)
2. Ask Claude to spawn a sub-agent that makes multiple sequential tool calls (not batched). For example: "spawn a sub-agent and have it read 5 files one at a time, summarizing each before reading the next"
3. While the sub-agent is running, switch focus between the main conversation and the sub-agent panel (mouse click in fullscreen mode, keyboard navigation in classic mode)
4. Observe the sub-agent's expanded output panel

Actual rendering in fullscreen mode (top to bottom):

⏺ File 2 summary: ...

  Read 1 file 

⏺ File 3 summary: ...

  Read 1 file 

⏺ File 1 summary: ...
  
  Read 1 file 

❯ Read the following 5 files one at a time...  <-- initial prompt appears in the MIDDLE

⏺ Thinking for 2s, reading 1 file…            <-- first action appears at the bottom
  ⎿  go.mod

Actual rendering in classic/default mode (top to bottom):

  Read 1 file (ctrl+o to expand)

⏺ CLAUDE.md summary...

  Read 1 file (ctrl+o to expand)

❯ Read the following 5 files one at a time...  <-- initial prompt in the MIDDLE

  Thought for 3s, read 1 file (ctrl+o to expand)

⏺ go.mod summary...                           <-- first result at the bottom

In both cases, the initial prompt should be at the top with all subsequent messages below in chronological order.

Error Messages/Logs

No error messages — purely a rendering/layout issue.

Claude Model

Opus

Is this a regression?

I don't know

Claude Code Version

2.1.159 (Claude Code)

Platform

Other

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

• macOS 26.5 (Darwin 25.5.0)
• Reproduces in both fullscreen and classic TUI renderers
• In fullscreen mode: switching via mouse click; in classic mode: switching via keyboard navigation
• The bug is about the inline sub-agent expansion in the main CLI conversation, not the separate claude agents dashboard view
• Possibly related to #59750 (Windows agent TUI elements rendering in wrong vertical positions), though that issue is about the claude agents view specifically