Custom statusline compressed to 1 char/line when notification banners are active

Bug description

When a UI notification banner is active (e.g., npm-deprecation-warning, programdata-deprecation-warning), custom statusline output is compressed vertically — each character renders on its own line, making the statusline completely unreadable.

This happens because the footer layout uses flex with different shrink behaviors:

• Statusline container (H6): flexShrink: 1 — shrinks to accommodate notifications
• Notifications container (Y6): flexShrink: 0 — never shrinks

When a multi-line notification banner appears, the statusline area shrinks to near-zero height, causing the text to wrap character-by-character.

Steps to reproduce

1. Install Claude Code via npm (triggers npm-deprecation-warning banner on startup)
2. Configure a custom statusline in ~/.claude/settings.json:

``json
{
"statusLine": {
"type": "command",
"command": "bash ~/.claude/statusline.sh"
}
}
``

1. Start a new Claude Code session
2. Observe the statusline is compressed/unreadable while the notification banner is visible (~15s)
3. After the banner auto-dismisses, the statusline renders correctly

Expected behavior

The statusline should remain readable when notification banners are active. Either:

• Both containers should share available space proportionally
• The statusline should have flexShrink: 0 (or a minimum height)
• Users should have a way to disable/suppress specific startup banners

Actual behavior

The statusline text wraps to 1 character per line, becoming completely unreadable. This persists for the duration of the notification banner (typically 15 seconds for startup banners).

!Image

Environment

• Claude Code version: 2.1.49
• Platform: Windows 11 (Git Bash)
• Terminal: Windows Terminal
• Install method: npm

Additional context

• The addNotification system that displays these banners does NOT go through the Notification hook system, so there's no way to detect or suppress them from the statusline script
• Identified ~20 notification types that can trigger this, most impactful being startup banners with 15s timeout
• The statusLine.padding setting only adds horizontal spacing and does not help
• No existing setting to disable specific notification banners
• Related issues: #21867, #22222, #23623 (all involve statusline layout compression in different scenarios)