fix: generate IDs for HTML headings without id attribute (#36233)

This PR fixes #27383 where HTML headings like `<h1>Title</h1>` in
markdown files would have empty permalink anchors

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/html_node_test.go

@@ -0,0 +1,104 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestProcessNodeAttrID_HTMLHeadingWithoutID(t *testing.T) {
+	// Test that HTML headings without id get an auto-generated id from their text content
+	// when EnableHeadingIDGeneration is true (for repo files and wiki pages)
+	testCases := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "h1 without id",
+			input:    `<h1>Heading without ID</h1>`,
+			expected: `<h1 id="user-content-heading-without-id">Heading without ID</h1>`,
+		},
+		{
+			name:     "h2 without id",
+			input:    `<h2>Another Heading</h2>`,
+			expected: `<h2 id="user-content-another-heading">Another Heading</h2>`,
+		},
+		{
+			name:     "h3 without id",
+			input:    `<h3>Third Level</h3>`,
+			expected: `<h3 id="user-content-third-level">Third Level</h3>`,
+		},
+		{
+			name:     "h1 with existing id should keep it",
+			input:    `<h1 id="my-custom-id">Heading with ID</h1>`,
+			expected: `<h1 id="user-content-my-custom-id">Heading with ID</h1>`,
+		},
+		{
+			name:     "h1 with user-content prefix should not double prefix",
+			input:    `<h1 id="user-content-already-prefixed">Already Prefixed</h1>`,
+			expected: `<h1 id="user-content-already-prefixed">Already Prefixed</h1>`,
+		},
+		{
+			name:     "heading with special characters",
+			input:    `<h1>What is Wine Staging?</h1>`,
+			expected: `<h1 id="user-content-what-is-wine-staging">What is Wine Staging?</h1>`,
+		},
+		{
+			name:     "heading with nested elements",
+			input:    `<h2><strong>Bold</strong> and <em>Italic</em></h2>`,
+			expected: `<h2 id="user-content-bold-and-italic"><strong>Bold</strong> and <em>Italic</em></h2>`,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var result strings.Builder
+			ctx := NewTestRenderContext().WithEnableHeadingIDGeneration(true)
+			err := PostProcessDefault(ctx, strings.NewReader(tc.input), &result)
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expected, strings.TrimSpace(result.String()))
+		})
+	}
+}
+
+func TestProcessNodeAttrID_SkipHeadingIDForComments(t *testing.T) {
+	// Test that HTML headings in comment-like contexts (issue comments)
+	// do NOT get auto-generated IDs to avoid duplicate IDs on pages with multiple documents.
+	// This is controlled by EnableHeadingIDGeneration which defaults to false.
+	testCases := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "h1 without id in comment context",
+			input:    `<h1>Heading without ID</h1>`,
+			expected: `<h1>Heading without ID</h1>`,
+		},
+		{
+			name:     "h2 without id in comment context",
+			input:    `<h2>Another Heading</h2>`,
+			expected: `<h2>Another Heading</h2>`,
+		},
+		{
+			name:     "h1 with existing id should still be prefixed",
+			input:    `<h1 id="my-custom-id">Heading with ID</h1>`,
+			expected: `<h1 id="user-content-my-custom-id">Heading with ID</h1>`,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var result strings.Builder
+			// Default context without EnableHeadingIDGeneration (simulates comment rendering)
+			err := PostProcessDefault(NewTestRenderContext(), strings.NewReader(tc.input), &result)
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expected, strings.TrimSpace(result.String()))
+		})
+	}
+}