feat: Copy Mode for OpenCode

[thuanpham582002]

Summary

OpenCode currently lacks a proper copy mode, making it difficult for users to selectively copy text from chat
messages, code blocks, and other content displayed in the viewport. While basic message copying exists (ctrl+x y), users need vim/tmux-like copy mode for precise text selection and copying.

Problem Statement

Current Limitations

1. No Keyboard-based Text Selection: Users cannot select text using keyboard only
2. Limited Copy Functionality: Only entire message copying is available (ctrl+x y)
3. Incompatible with tmux Copy Mode: OpenCode runs as a TUI application but doesn't integrate with tmux's copy
   mode
4. Poor UX for Code Review: Cannot selectively copy code snippets, error messages, or specific parts of
   responses
5. Mouse Dependency: Users must rely on mouse for text selection, which is inefficient in terminal
   environments

User Impact

• Developers: Cannot quickly copy code snippets from AI responses
• Power Users: Keyboard-driven workflow is broken when needing to copy text
• Terminal Enthusiasts: Expect vim/tmux-like navigation and selection
• Accessibility: Mouse-dependent selection is not accessible to all users

Why Copy Mode is Essential

1. Terminal Workflow Compatibility

OpenCode is a TUI (Terminal User Interface) application that runs in terminal emulators. Terminal users expect:

• Vim-like navigation and selection
• Keyboard-driven text manipulation
• Integration with existing terminal tools

2. tmux Integration Problem

When OpenCode runs inside tmux:

• tmux's copy mode (prefix+[) doesn't work properly, it only can select and copy one page that currently visible of opencode.

3. Pager-like Applications Need Copy Mode

OpenCode functions as a pager for chat messages and code:

• Similar to less, more, or vim
• All major pagers have copy/select functionality
• Users expect to be able to select and copy content

4. Developer Productivity

Developers using OpenCode need to:

• Copy code snippets from AI responses
• Select error messages for debugging
• Extract specific parts of long responses
• Maintain keyboard-driven workflow

What Already Exists (✅)

1. Selection System:

   type selection struct {
       startX int
       endX   int
       startY int
       endY   int
   }

• Mouse-based selection is implemented
• Coordinate tracking works correctly

2. Highlight System:
```go
type highlightInfo struct {
    lineStart, lineEnd int
    lines map[int][2]int
}

• Viewport has robust highlighting
• Multi-line highlighting supported
• SetHighlights(), HighlightNext(), HighlightPrevious() available
3. Navigation Keymap:

type KeyMap struct {
    PageDown, PageUp key.Binding
    HalfPageUp, HalfPageDown key.Binding
    Down, Up key.Binding
    Left, Right key.Binding
}

• Vim-like keys already implemented: j/k, h/l, f/b, d/u
• Navigation system is mature
4. Clipboard Integration:
• cmds = append(cmds, app.SetClipboard(lastTextPart.Text))
• app.SetClipboard() function exists
• Toast notification system available

[github-actions]

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

• No way to select or copy text, either in the input field or from the output messages #2114: Directly addresses the core problem - no way to select or copy text from output messages
• Add a /copy command to copy latest message (or allow scrolling while selecting text) #2118: Requests a /copy command and better text selection with scrolling capabilities
• Mouse text selection has unexpected behavior in terminal #1210: Addresses mouse text selection issues and specifically mentions wanting vim/tmux-like behavior
• [Question] how to scroll by keyboard? #862: Requested keyboard shortcuts for scrolling (Ctrl+d, Ctrl+u) and mentions tmux usage
• Standard keyboard cursor navigation (Home/End key, ctrl+left/right key) #2649: Requests improved keyboard navigation (Home/End, ctrl+left/right keys)

Feel free to ignore if none of these address your specific case.

[rekram1-node]

yeah copying could be better, tui is being rewritten currently so features like this can best be addressed after the fact.

Ultimately all ux goes through @adamdotdevin

[jonahbenton]

Voting +1. Would also like selection by mouse and keyboard navigation.