Improve MarkdownDef formats

[lukemelia]

Summary

• Isolated: Renders full markdown content via MarkdownTemplate (headings, lists, code blocks, tables, syntax highlighting); falls back to title when content is empty
• Embedded: Title header + rendered markdown content with max-height: 200px and gradient fade-out mask to indicate more content
• Fitted: MarkdownIcon + title + plain-text excerpt with container queries adapting layout for portrait tall/short, landscape, and very small (≤57px) sizes
• Atom: Inline 16px MarkdownIcon + title with ellipsis overflow
• Deletes the now-unused markdown-file-preview.gts (single component previously shared across all formats)

Screen.Recording.2026-02-19.at.6.02.29.PM.mov

Test plan

• Verify build succeeds (template lint passes, no dangling imports)
• Existing extraction tests in markdown-file-def-test.gts pass unchanged
• Manual visual verification: open a markdown file in isolated, embedded, fitted, and atom contexts
• Verify fitted format adapts correctly at different container sizes

🤖 Generated with Claude Code

[github-actions]

Host Test Results

    1 files  ±0      1 suites  ±0   1h 42m 1s ⏱️ -57s
1 856 tests +2  1 842 ✅ +2  14 💤 ±0  0 ❌ ±0 
1 871 runs  +2  1 857 ✅ +2  14 💤 ±0  0 ❌ ±0 

Results for commit d19686c. ± Comparison against base commit 975718c.

♻️ This comment has been updated with latest results.

[backspace]

Maybe this is a separate issue but I’d expect changes to the Markdown to be reflected in the preview, I had to refresh to see them:

[Copilot]

Pull request overview

This PR refactors MarkdownDef’s presentation formats so markdown files render richer previews across isolated/embedded/fitted/atom contexts, and removes the previously shared preview component.

Changes:

• Replace the single shared MarkdownFilePreview with dedicated Isolated, Embedded, Fitted, Atom, and Head components inside markdown-file-def.gts.
• Render full markdown via MarkdownTemplate for isolated/embedded formats (with embedded height clamping + fade mask).
• Remove the unused packages/base/markdown-file-preview.gts component and document a .gts regex/content-tag parsing gotcha in AGENTS.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/base/markdown-file-preview.gts	Deleted the old shared preview component now replaced by per-format implementations.
packages/base/markdown-file-def.gts	Adds per-format components (isolated/embedded/fitted/atom/head), uses MarkdownTemplate, updates excerpt parsing/truncation.
AGENTS.md	Documents content-tag lexer pitfalls with certain regex literals in .gts files and recommended workarounds.

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[Copilot]

FENCED_CODE_RE is still a regex literal containing backticks (```), but the new guidance/comment indicates content-tag can misparse backticks in regex literals in .gts files. To avoid reintroducing the same parsing failure, consider defining this via `new RegExp()` (string pattern) as well, or otherwise ensure no backticks appear in regex literals in this file.

Suggested change

	const FENCED_CODE_RE = /```[\s\S]*?```/g;
	const FENCED_CODE_RE = new RegExp('```[\\s\\S]*?```', 'g');

[Copilot]

The title getter logic is duplicated across multiple format components (Isolated/Embedded/Fitted/Atom/Head). This duplication makes it easy for the formats to drift (e.g., changing fallback text in one place but not the others). Consider extracting a shared helper (or a small base class) to compute the title once and reuse it across these components.

Suggested change

	class Isolated extends Component<typeof MarkdownDef> {
	get title() {
	return (
	this.args.model?.title ?? this.args.model?.name ?? 'Untitled markdown'
	);
	function computeMarkdownTitle(
	model: { title?: string | null; name?: string | null } | null | undefined,
	): string {
	return model?.title ?? model?.name ?? 'Untitled markdown';
	}
	class Isolated extends Component<typeof MarkdownDef> {
	get title() {
	return computeMarkdownTitle(this.args.model);

[backspace]

I tried this locally and the previews look good across the different formats, I assume the lack of live updating is an existing problem

[habdelra]

I tried this locally and the previews look good across the different formats, I assume the lack of live updating is an existing problem

I tried this locally and the previews look good across the different formats, I assume the lack of live updating is an existing problem

the not live updating makes me wonder how the store is handling realm update events for file-defs. perhaps there is a bug there. we def need test coverage around that scenario.