Code-fenced directory trees in Markdown cause rendering and copy issues

Category
Bug Report

Environment

• OS: Ubuntu 25.10 (Questing Quokka)

• Browser: Mozilla Firefox 145.0.2

• Site: https://t3.chat

Disclosure
This report was formulated by an AI assistant (GPT-5) on my request.
https://t3.chat/chat/7e77b4e8-57fb-4af4-9175-9270d443a5fe

Steps to reproduce

1. Start a new chat on t3.chat.

2. Send a Markdown README inside a single fenced code block, for example using ```markdown as the fence.

3. Include a “Package structure” section that looks like an ASCII directory tree with inline comments (file names on the left, comments on the right). Example content to put inside the fence:
   mapgen/
   init.py # package marker
   constants.py # styles and tunables
   geo_io.py # projection/clip/path helpers + YAML/raster + SVG DOM
   core_render.py # canvas/bbox sizing + map/grid/markers
   scenes.py # scene discovery + mask/crop + export
   pipeline.py # orchestrator
   cli.py # argparse wrapper

4. Send the message.

Expected behavior

• The message renders as a normal code block.

• The entire content stays intact and is copyable verbatim (no truncation, no reflow, no dropped lines).

Actual behavior

• The UI appears to get confused by the code-fenced ASCII tree:

  • Portions of the block are truncated or not displayed.

  • Copy/paste yields incomplete or modified text (spacing/lines changed).

  • Reposting similar blocks sometimes fails again, inconsistently.

• When the same README is reformatted (no ASCII tree, just bullet points), it works reliably.

Impact

• Makes it hard to share READMEs or snippets that include directory trees.

• Forces users to avoid common documentation patterns or use workarounds (base64, bullets, multiple small blocks).

Workarounds observed

• Replace the tree with a bullet list of files and descriptions.

• Split content into smaller fenced blocks.

• Encode the content as base64 and share that instead.

Notes

• The issue seems triggered by a code-fenced block that resembles shell tree output with inline comments.

• It may be an overzealous heuristic that tries to re-interpret or sanitize tree-like code within fenced blocks.

Proposed fixes

• Treat all fenced code blocks as inert text (no tree/CLI heuristics inside fences).

• Provide a “Copy raw” button that copies the exact underlying text.

• If sanitization is required, avoid applying it to fenced code blocks.

Attachments

• I can provide a screenshot showing the composition UI and the problematic section upon request.