[Windows Terminal] Native mermaid/inline image rendering via Sixel (WT_SESSION detection missing)

Problem

On Windows Terminal, inline images (mermaid diagrams, LaTeX, image attachments) fall back to the Unicode half-block text renderer with the note
│ "Your terminal cannot render inline images; showing a text fallback." even though Windows Terminal 1.22+ natively supports Sixel graphics,
│ including passthrough of image sequences from console apps through ConPTY.

Root cause

crates/jcode-tui-mermaid/src/mermaid_runtime.rs::infer_protocol_from_env recognizes wezterm / kitty / ghostty / handterm / iterm and TERM containing "sixel", but Windows Terminal sets none of these (it exposes only WT_SESSION, WT_PROFILE_ID, WT_WINDOW_ID), so the picker stays on the Halfblocks fallback. The CLI-side detection in crates/jcode-terminal-image/src/display.rs::ImageProtocol::detect has the same gap.

Verified end-to-end

• Environment: Windows Terminal Preview 1.25 (bundled OpenConsole), Windows 10/11.
• Method: force the Sixel branch with set TERM=sixel-256color && jcode, then render a flowchart.
• Result: the flowchart renders natively via Sixel, clearly readable, with no fallback note. jcode logs:
  Mermaid picker init: mode=Fast multiplexer=none env_protocol=Some(Sixel).
  The same flow without the TERM override logs env_protocol=None and shows the half-block fallback.
• Minor quality note: node text is slightly blurry due to Windows Terminal scaling Sixel images to whole-cell boundaries. Likely improvable by rendering at measured pixel sizes (mmdr size API), not a blocker.

Proposed change

Add Windows Terminal detection to the fast env-based path, e.g. in infer_protocol_from_env:

if WT_SESSION is set -> return ProtocolType::Sixel and mirror it in the CLI crate's ImageProtocol::detect (without the ImageMagick requirement used by its Sixel path, since ratatui-image encodes Sixel itself).

Considerations:

• Requires Windows Terminal >= 1.22 (Sixel support; older versions would render raw Sixel bytes). Consider gating on a capability probe or documenting the minimum version.
• Verify inline mermaid diagrams in chat and side panel under WT 1.22+: placeholder geometry, scroll/resize behavior, session restore.

Workarounds today

set TERM=sixel-256color && jcode in Windows Terminal enables native rendering immediately.

• Alacritty cannot be fixed app-side: it supports no image protocol at all.

Related

• Halfblock fallback note: TERMINAL_IMAGE_FALLBACK_NOTE in crates/jcode-tui-mermaid/src/mermaid_content.rs

Start with infer_protocol_from_env in crates/jcode-tui-mermaid/src/mermaid_runtime.rs and ImageProtocol::detect in crates/jcode-terminal-image/src/display.rs, then review the existing Sixel and environment detection paths. Verify Windows Terminal 1.22+ renders inline mermaid images without the half-block fallback, including chat and side-panel resize, scrolling, and session restore behavior.