Desktop app File viewer: clicking an .html file shows source only — it used to open a rendered preview

Summary

Clicking a generated .html file in the Claude Code desktop app opens the File viewer showing syntax-highlighted source only. There is no rendered-preview option. This is a regression: HTML files used to open as a rendered preview when clicked; at some point (noticed by 2026-08-13) they started opening as source instead.

Workflow affected

Claude generates self-contained HTML deliverables (QA walkthroughs / reports with inline CSS and base64-embedded screenshots) and sends them as file cards in the conversation. Files delivered with display: "render" do render in the side panel once — but any subsequent click on the file card (or the file in the tree) lands in the source viewer. Every re-visit requires asking Claude to run open -a "Google Chrome" <path>, which defeats the point of the in-app viewer.

Expected

Clicking an .html file renders it (as the preview pane does for a URL), ideally with a Source/Preview toggle. This is how it previously behaved.

Actual

Line-numbered, syntax-highlighted source, with no way to switch to a rendered view.

Environment

• Claude Code desktop app, macOS (Darwin 25.5.0)
• Observed 2026-08-13; the previous (rendered) behavior was working within recent weeks
• Repro file: a ~2.5 MB self-contained walkthrough.html (inline CSS, base64 PNG data URIs) inside a git worktree

Screenshot to follow in a comment (shows the file card on the left and the source-only File viewer on the right).