[FEATURE] TUI: Collapsible right panel and multi-window support

[x-thooh]

Problem

Currently, OpenCode TUI only supports toggling the left session sidebar (via ctrl+x b). There is no way to:

1. Collapse/expand the right panel - Users cannot hide tool execution details, file previews, or other right-side content to maximize the chat area
2. Multi-window/split pane layout - Users cannot have multiple sessions visible side-by-side within the same TUI instance

Proposed Solution

1. Collapsible Right Panel

Add a keybind (e.g., ctrl+x r) to toggle the visibility of the right panel, similar to how sidebar_toggle works for the left sidebar.

2. Multi-window / Split Pane

Add support for:

• Horizontal split (side-by-side sessions)
• Vertical split (stacked sessions)
• Tab management within splits

This could follow patterns from tools like tmux or Vim splits.

Alternatives Considered

Currently, users can:

• Use tmux to run multiple OpenCode instances in splits (workaround)
• Use VS Code extension for split terminal views

But native support would provide better UX and integration.

Additional Context

• OpenCode version: 1.3.2
• Related issues: [FEATURE]: Multi-window/tabs for desktop #14657 (desktop multi-window), [FEATURE]: Session & Subagent Tabs in the TUI for better orchestration #17838 (TUI tabs)

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• Missing the required verification checkbox: "I have verified this feature I'm about to request hasn't been suggested before" (required by the Feature Request template)

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Multi-Grid View for Tiling Sessions #18287: Multi-Grid View for Tiling Sessions — overlaps with the split pane/multi-window support aspect of this request

For keybind-related issues, please also check our pinned keybinds documentation: #4997

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

This issue has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

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