feat(opencode): enhanced markdown renderer with tables, themes, and syntax highlighting

[ryanwyler]

Summary

Custom markdown renderer that works alongside upstream's experimental markdown flag. When OPENCODE_EXPERIMENTAL_MARKDOWN is set, upstream's <markdown> element is used unchanged. When unset (default), a hybrid renderer provides enhanced markdown formatting with proper table rendering, tree-sitter syntax highlighting, and theme support.

Single commit, additive only - no upstream code removed or modified beyond minimal integration points.

Features

• Box-drawing table rendering with cell word wrap and unicode-aware widths
• Tree-sitter syntax highlighting for code blocks via native <code> element
• Task list checkbox rendering (- [x] / - [ ])
• Strikethrough support (~~text~~)
• Header background panels for h1/h2
• Diff code block coloring (+/- lines)
• CLI markdown rendering via UI.markdown() with theme support
• Theme loader supporting all 33 upstream themes

Files Changed

File	Change	Description
cli/markdown-renderer.ts	New (1310 lines)	Core rendering engine
cli/theme-loader.ts	New (127 lines)	Loads all 33 upstream themes, converts to renderer format
cli/cmd/tui/routes/session/index.tsx	+113 lines	TUI integration: TextPart, Prose, CodeBlock, MarkdownDiff components
cli/cmd/run.ts	+12 lines	CLI theme loading for non-interactive mode
cli/ui.ts	+5 lines	UI.markdown() now calls custom renderer with theme support

Total: 5 files, 1552 additions, 15 deletions

Architecture

The renderer integrates with upstream's existing OPENCODE_EXPERIMENTAL_MARKDOWN flag:

TextPart component
  |
  +-- OPENCODE_EXPERIMENTAL_MARKDOWN=1
  |     -> upstream's <markdown> element (untouched)
  |
  +-- OPENCODE_EXPERIMENTAL_MARKDOWN unset (default)
        -> parseMarkdownSegments() splits markdown into text + code segments
             |
             +-- Text segments -> renderMarkdownThemedStyled() -> StyledText -> <text>
             +-- Code segments -> tree-sitter via <code filetype={lang}> (native OpenTUI)

CLI mode (opencode run):

User config -> loadTheme() -> MarkdownTheme
Markdown -> renderMarkdownThemedStyled() -> TextChunks -> ANSI -> stdout

Table Rendering

Before (plain text):          After (box-drawing):
| Header | Value |            +---------+-------+
| A      | B     |            | Header  | Value |
                              +---------+-------+
                              | A       | B     |
                              +---------+-------+

Related Issues

• Closes Markdown tables display as plain text instead of formatted tables anomalyco/opencode#3845 - Markdown tables display as plain text instead of formatted tables
• Closes Markdown table formatting anomalyco/opencode#7671 - Markdown table formatting
• Closes [FEATURE]: Support Table Rendering in Markdown Format anomalyco/opencode#4988 - [FEATURE]: Support Table Rendering in Markdown Format
• Closes markdown user messages aren't formatted anomalyco/opencode#5675 - markdown user messages aren't formatted

[github-actions]

Thanks for your contribution!

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.