tui: main prompt flashes while scrolling through terminal history

Summary

While scrolling upward through a long opencode session, the main input prompt briefly flashes on screen even when the current scroll position is far away from it.

Environment

• opencode version: 0.0.0-beta-18414
• OS: macOS, Darwin 25.6.0 (arm64)
• Terminal: Zed 1.17.2 integrated terminal (TERM_PROGRAM=zed, TERM=xterm-256color, COLORTERM=truecolor)
• Shell: /bin/zsh
• Install/channel: opencode 2 beta
• Active plugins: None found

Reproduction

1. Open opencode in Zed's integrated terminal.
2. Build up enough session output that the main input prompt is far below the visible terminal history.
3. Scroll upward using a mouse wheel or trackpad.
4. Observe the screen while scrolling.

Expected Behavior

Only the terminal history at the current scroll position should be rendered. The main input prompt should not appear when it is outside the visible region.

Actual Behavior

The main input prompt flashes on screen for a split second while scrolling upward, even when the current position is far away from the prompt.

The issue occurs deterministically when scrolling upward. It has only been reproduced in Zed's integrated terminal.

Additional Context

• No known update or configuration change preceded the issue.
• No workaround has been tried.
• No active plugins were found.
• A screen recording is available and will be attached.