Improve shortcode indentation handling

* Record the leading whitespace (tabs, spaces) before the shortcode when parsing the page.
* Apply that indentation to the rendered result of shortcodes without inner content (where the user will apply indentation).

Fixes #9946

hugolib/shortcode.go

@@ -170,6 +170,8 @@ type shortcode struct {
 	ordinal   int
 	err       error
 
+	indentation string // indentation from source.
+
 	info   tpl.Info       // One of the output formats (arbitrary)
 	templs []tpl.Template // All output formats
 
@@ -398,6 +400,22 @@ func renderShortcode(
 		return "", false, fe
 	}
 
+	if len(sc.inner) == 0 && len(sc.indentation) > 0 {
+		b := bp.GetBuffer()
+		i := 0
+		text.VisitLinesAfter(result, func(line string) {
+			// The first line is correctly indented.
+			if i > 0 {
+				b.WriteString(sc.indentation)
+			}
+			i++
+			b.WriteString(line)
+		})
+
+		result = b.String()
+		bp.PutBuffer(b)
+	}
+
 	return result, hasVariants, err
 }
 
@@ -447,6 +465,15 @@ func (s *shortcodeHandler) extractShortcode(ordinal, level int, pt *pageparser.I
 	}
 	sc := &shortcode{ordinal: ordinal}
 
+	// Back up one to identify any indentation.
+	if pt.Pos() > 0 {
+		pt.Backup()
+		item := pt.Next()
+		if item.IsIndentation() {
+			sc.indentation = string(item.Val)
+		}
+	}
+
 	cnt := 0
 	nestedOrdinal := 0
 	nextLevel := level + 1

hugolib/shortcode_test.go

@@ -942,3 +942,76 @@ title: "p1"
 	`)
 
 }
+
+func TestShortcodePreserveIndentation(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+-- content/p1.md --
+---
+title: "p1"
+---
+
+## List With Indented Shortcodes
+
+1. List 1
+    {{% mark1 %}}
+	1. Item Mark1 1
+	1. Item Mark1 2
+	{{% mark2 %}}
+	{{% /mark1 %}}
+-- layouts/shortcodes/mark1.md --
+{{ .Inner }}
+-- layouts/shortcodes/mark2.md --
+1. Item Mark2 1
+1. Item Mark2 2
+   1. Item Mark2 2-1
+1. Item Mark2 3
+-- layouts/_default/single.html --
+{{ .Content }}
+`
+
+	b := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			Running:     true,
+		},
+	).Build()
+
+	b.AssertFileContent("public/p1/index.html", "<ol>\n<li>\n<p>List 1</p>\n<ol>\n<li>Item Mark1 1</li>\n<li>Item Mark1 2</li>\n<li>Item Mark2 1</li>\n<li>Item Mark2 2\n<ol>\n<li>Item Mark2 2-1</li>\n</ol>\n</li>\n<li>Item Mark2 3</li>\n</ol>\n</li>\n</ol>")
+
+}
+
+func TestShortcodeCodeblockIndent(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+-- content/p1.md --
+---
+title: "p1"
+---
+
+## Code block
+
+    {{% code %}}
+
+-- layouts/shortcodes/code.md --
+echo "foo";
+-- layouts/_default/single.html --
+{{ .Content }}
+`
+
+	b := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			Running:     true,
+		},
+	).Build()
+
+	b.AssertFileContent("public/p1/index.html", "<pre><code>echo &quot;foo&quot;;\n</code></pre>")
+
+}