Scrollback in Zellij Pane Broken for OpenCode, But Works for Claude Code #3020

Describe the bug

When running AI code assistants in Zellij (e.g., Codex, OpenCode, Claude Code, etc), I’m unable to scroll up and down in the scrollback buffer when using OpenCode. However, this works correctly for Claude Code’s terminal output. In OpenCode sessions, attempting to scroll does not reveal previous output as expected. Claude Code does allow full Zellij scrollback, so the bug is likely due to differences in how OpenCode manages terminal output or buffer modes.

Steps to reproduce

Launch Zellij. Open a pane and start OpenCode (with output heavy commands).
Attempt to scroll back using Zellij’s scroll/copy mode or the mouse.
Repeat with Claude Code in a separate pane for comparison.

Expected behavior

Output from both apps should be scrollable in Zellij’s scrollback buffer using the standard scroll/copy mode, PgUp/PgDn, and mouse wheel.
Actual behavior
OpenCode output is not accessible in scrollback; scrolling fails or is incomplete.
Claude Code output is fully accessible in the scrollback buffer.

Environment

OS: WSL 2 on Windows 11
Zellij Version: zellij 0.42.2

Additional Context

This issue affects productivity when using Zellij with OpenCode. It occurs even after enabling mouse mode in Zellij and using scroll/copy shortcuts. Suspect this may be related to use of alternate screen buffer or specific terminal escape sequences in OpenCode.