tui: transparent terminal shows black bar below prompt

Summary

When terminal transparency is enabled, a black bar appears beneath the prompt input box in both OpenCode v1 and v2. The bar does not blend naturally with the transparent background in either version.

Environment

• opencode version: v1.18.19 and v2.0.0-beta-17728
• OS: Linux 7.1.8-1-cachyos, x86_64
• Terminal: Alacritty with Tokyo Night theme
• Shell: fish
• Install/channel: v1 stable and v2 beta
• Active plugins: superpowers and Graphify

Reproduction

1. Configure Alacritty with window.opacity = 0.80, window.dynamic_padding = true, window.blur = true, and colors.transparent_background_colors = true.
2. Launch OpenCode v1 or v2 with the Tokyo Night theme.
3. Focus the prompt input box.

Expected Behavior

The prompt input area should blend naturally with the transparent terminal background.

Actual Behavior

A black bar appears at the bottom of the prompt input box in both versions. It does not blend naturally with the transparent background and breaks the transparency aesthetic.

Additional Context

If the bar is intended to provide shadow or 3D depth, it is too opaque and prominent in both versions.