Bottom UI elements flicker/reorder during spinner updates

Description

When a background agent is running (spinner active), the bottom section UI elements rapidly swap ordering between two distinct layouts on each spinner redraw. This causes visible flickering/reflowing of the entire bottom section.

Steps to Reproduce

1. Run a task that spawns subagents (e.g., TeamCreate with parallel agents)
2. While a spinner is active (e.g., "Fluttering..." status), observe the bottom UI elements

Expected Behavior

Bottom elements should maintain a stable ordering during spinner updates.

Actual Behavior

Elements reorder between two layouts on each spinner tick, causing visible flicker. The affected elements are:

Layout A (intermittent):

1. Agent completion notification
2. Spinner status line
3. Feedback prompt
4. Input box

Layout B (intermittent):

1. Input box
2. Spinner status line
3. Feedback prompt
4. Agent completion notification

The ordering flips back and forth rapidly with each spinner frame.

Environment

• macOS 15.7.4 (Darwin 24.6.0)
• Terminal: 164x48
• Mac model: Mac16,7

Evidence

I have a screen recording demonstrating the issue but cannot share it publicly. Happy to share it privately with the Anthropic team — please reach out and I can provide it.