Slash command autocomplete popup capped at ~3–5 commands regardless of terminal height

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

The / slash command autocomplete popup is hard-capped to ~3–5 visible commands regardless of terminal height. Every other picker in the CLI (/help, /config, /model, /plugin, /skills) correctly uses the available terminal space. The / popup is the outlier.

What Should Happen?

The popup should expand to fit the available rows above the prompt, the same way the other modal pickers do. With 60+ built-in commands plus user skills, plugin commands, and MCP prompts, a 3–5 command window is effectively unusable for both finding a known command and browsing what's available. Makes learning what's available, and searching for known commands, just awkward.

Steps to Reproduce

1. Open Claude Code in a maximized terminal window (any terminal, any OS).
2. Type / at the prompt.
3. Observe the popup renders with only 3–5 visible commands, with the rest of the screen empty above it.
4. For comparison, run /help or /config and observe those pickers correctly use the full terminal height.

Additional Context

• /tui fullscreen does not change the behavior, so this isn't a renderer-mode issue.
• Behavior suggests a hardcoded maxVisibleRows constant in the popup component, separate from the modal picker code path.
• This is closely related in spirit to #32915 (Add to Project dropdown capped at ~8 items despite scrollbar) — same pattern of hardcoded picker heights that don't respect available space.

Environment

• Claude Code version: 2.1.121
• OS: macOS 26.5
• Terminal: tested in standard macOS terminal emulator
• Renderer mode: tested in both inline and /tui fullscreen