[BUG] Slash-command menu: large blank gap pushes suggestion list off-screen (Windows, v2.1.219)

### What's Wrong?

When invoking the slash-command menu (typing `/` at the prompt), a large blank vertical gap renders between the welcome banner and the command-suggestion list, pushing the list toward the bottom of the terminal. As a result, only the last 1-2 matching commands/skills are visible — the rest of the list is not displayed and does not appear to be reachable by scrolling.

This looks like the same root-cause layout bug reported in #66089 and #66191 (large blank gap between welcome box and prompt box on launch, caused by the v2.1.168 "bottom-anchored input" layout change miscalculating available terminal height on some OS/terminal combinations). Those issues were auto-closed by the stale-bot for inactivity, not because the underlying bug was fixed — and this report shows the same symptom still present at v2.1.219, manifesting in the slash-command completion list rather than at startup.

### Environment

- OS: Windows 11 Home, build 10.0.26200
- Claude Code version: 2.1.219
- Node: 24.13.1
- Terminal: Windows Terminal (dedicated "Claude Code" pinned profile/tab)
- Reproduces identically in: that Windows Terminal profile, plain `cmd.exe`, PowerShell, and Git Bash — both standalone and inside Cursor's integrated terminal

### Steps to Reproduce

1. Open a terminal on Windows (reproduces across cmd.exe, PowerShell, Git Bash, Windows Terminal, and Cursor's integrated terminal)
2. `cd` into a project directory containing custom skills/slash-commands
3. Run `claude`
4. At the prompt, type `/`
5. Observe the command-suggestion list

### Expected Behavior

The full list of matching slash commands/skills renders directly below the input, scrollable if it exceeds the terminal height.

### Actual Behavior

A large empty/blank region renders above the suggestion list, and only the last couple of entries are visible — the rest of the list is effectively hidden, not just scrolled.

### Is this a regression?

Yes — matches the layout regression introduced around v2.1.168 described in #66089, still present at v2.1.219.

### Related issues

- #66089
- #66191

Start by reproducing the slash-command menu in Claude Code by running `claude` in a Windows terminal and typing `/`; compare cmd.exe, PowerShell, Git Bash, Windows Terminal, and Cursor's integrated terminal. Read the layout behavior associated with the bottom-anchored input change described in #66089 and #66191. Done means the full matching list appears directly below the input and remains scrollable when it exceeds the terminal height.

Written by the indexing model from the issue text.