blockmap: a lazy-continuation admonition keeps its marker and its indent

CHANGELOG.md

@@ -9,6 +9,12 @@ At release time the section is retitled to the version and the prose lead is wri
 
 ## Unreleased
 
+### Fixed
+
+- A MkDocs admonition written without a blank line after its marker is no longer flattened into one paragraph, which stopped it rendering as a callout (#31, contributed by Karl Isenberg).
+  That spelling is a single paragraph to CommonMark, since the indented body is a lazy continuation rather than the code block the blank-line spelling produces.
+  The marker line is now immovable and the body carries the 4-space indent the extension requires, the same treatment a footnote definition already gets.
+
 ## v0.1.5 (2026-08-09)
 
 Narrower promises, structurally enforced.

internal/blockmap/blockmap.go

@@ -477,6 +477,18 @@ func build(p ast.Node, source []byte, inBlockquote bool, fmEnd int, precededByTa
 		// nesting), not a replacement for it.
 		contPrefix += "    "
 	}
+	if admonitionMarkerRE.MatchString(trimmed[0]) {
+		// A MkDocs admonition written without a blank line after its
+		// marker is one paragraph here: the indented body is a lazy
+		// continuation, not the code block the blank-line spelling
+		// produces. Joining the marker into the body destroys the
+		// callout, and so does dropping the body's indent, so the marker
+		// line is immovable and the body carries the 4-space indent the
+		// extension requires. Same treatment as the footnote definition
+		// above, for the same reason.
+		boundary[0] = true
+		contPrefix += "    "
+	}
 
 	return Paragraph{
 		Node:       p,

internal/blockmap/blockmap_internal_test.go

@@ -659,6 +659,27 @@ func TestBacktickInBareURLNeedsURLFirst(t *testing.T) {
 	}
 }
 
+// A MkDocs admonition written without a blank line after its marker is a
+// single paragraph: the indented body is a lazy continuation. Reflowing it
+// as ordinary prose joins the marker into the body and drops the indent,
+// and the callout stops being one.
+func TestLazyAdmonitionKeepsItsShape(t *testing.T) {
+	const src = "!!! tip \"Title here\"\n    The recommended shape is the v2 API at\n    a decomposed resource. Second sentence here.\n"
+	b := []byte(src)
+	doc := gm.New().Parser().Parse(text.NewReader(b))
+	paras := Paragraphs(doc, b)
+	if len(paras) != 1 {
+		t.Fatalf("got %d paragraphs, want 1", len(paras))
+	}
+	p := paras[0]
+	if !p.Boundary[0] {
+		t.Error("marker line is not a boundary, so reflow may join it into the body")
+	}
+	if p.ContPrefix != "    " {
+		t.Errorf("ContPrefix = %q, want the 4-space body indent", p.ContPrefix)
+	}
+}
+
 func TestParenGuardNarrowing(t *testing.T) {
 	cases := []struct {
 		name     string