[BUG] Status line renders vertically on macOS at startup until terminal width is detected

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?

Custom status line output renders vertically (1 character per line) for ~15 seconds at startup, then corrects itself and displays horizontally as expected.

The status line script outputs a short single line (~30 chars): [Model Name] ██████░░░░ 45%

This suggests the renderer is using a terminal width of 0 or 1 before the terminal reports its actual dimensions, causing every character to wrap.

Related to #13113 but different platform/trigger (macOS startup timing vs Windows Terminal).

Environment

• OS: macOS (Darwin 24.6.0)
• Terminal: default macOS terminal
• Status line: custom script via settings.json

Status line config

{
  "statusLine": {
    "type": "command",
    "command": "~/.claude/statusline.sh",
    "padding": 2
  }
}

What Should Happen?

The renderer should assume a reasonable minimum terminal width (e.g. 80 or 120 columns) when actual dimensions aren't available yet, rather than defaulting to 0/1 and wrapping every character.

What Actually Happens?

For ~15 seconds after launch, the status line text is rendered vertically with 1 character per line. After the terminal reports its real width, it snaps to the correct horizontal layout.