Fix markdown_to_html mangling content that starts with a blank line

CHANGELOG

@@ -2,6 +2,7 @@
 
  * Fix nested `block()` calls to resolve against the overriding template when a block rendered through `block(name, template)` calls `parent()`
  * Stop reporting a skipped test in `IntegrationTestCase` when there is no legacy test to run
+ * Fix `markdown_to_html` to strip the indentation shared by all lines instead of mangling content that starts with a blank line
  * Make the `IntegrationTestCase` and `NodeTestCase` test helpers compatible with PHPUnit 11
  * Cast printed expressions to string so values that cannot be converted to a string (arrays, non-`Stringable` objects, ...) report a usable stack trace at the print location
  * Make the `include()` function return a `Markup` object so an assigned result is not re-escaped when printed

extra/markdown-extra/MarkdownRuntime.php

@@ -22,11 +22,55 @@ class MarkdownRuntime
 
     public function convert(string $body): string
     {
-        // remove indentation
-        if ($white = substr($body, 0, strspn($body, " \t\r\n\0\x0B"))) {
-            $body = preg_replace("{^$white}m", '', $body);
+        return $this->converter->convert(self::stripCommonIndentation($body));
+    }
+
+    /**
+     * Removes the indentation shared by all non-blank lines.
+     *
+     * This lets authors indent a `{% apply markdown_to_html %}` block to match
+     * the surrounding template without that indentation leaking into Markdown
+     * (where leading whitespace is significant, e.g. code blocks).
+     */
+    private static function stripCommonIndentation(string $body): string
+    {
+        $lines = explode("\n", $body);
+
+        $indent = null;
+        foreach ($lines as $line) {
+            if ('' === trim($line)) {
+                continue;
+            }
+
+            $lineIndent = substr($line, 0, strspn($line, " \t"));
+            if (null === $indent) {
+                $indent = $lineIndent;
+                continue;
+            }
+
+            $max = min(\strlen($indent), \strlen($lineIndent));
+            $common = 0;
+            while ($common < $max && $indent[$common] === $lineIndent[$common]) {
+                ++$common;
+            }
+            $indent = substr($indent, 0, $common);
+
+            if ('' === $indent) {
+                return $body;
+            }
         }
 
-        return $this->converter->convert($body);
+        if (null === $indent || '' === $indent) {
+            return $body;
+        }
+
+        $length = \strlen($indent);
+        foreach ($lines as $i => $line) {
+            if (str_starts_with($line, $indent)) {
+                $lines[$i] = substr($line, $length);
+            }
+        }
+
+        return implode("\n", $lines);
     }
 }

extra/markdown-extra/Tests/FunctionalTest.php

@@ -17,6 +17,7 @@ use Twig\Extra\Markdown\DefaultMarkdown;
 use Twig\Extra\Markdown\ErusevMarkdown;
 use Twig\Extra\Markdown\LeagueMarkdown;
 use Twig\Extra\Markdown\MarkdownExtension;
+use Twig\Extra\Markdown\MarkdownInterface;
 use Twig\Extra\Markdown\MarkdownRuntime;
 use Twig\Extra\Markdown\MichelfMarkdown;
 use Twig\Loader\ArrayLoader;
@@ -77,6 +78,42 @@ EOF, "<h1>Hello</h1>\n+<p>Great!</p>"],
 {% endapply %}
 EOF, "<h1>Hello</h1>\n+<p>Great!</p>"],
             ["{{ include('html')|markdown_to_html }}", "<h1>Hello</h1>\n+<p>Great!</p>"],
+            [<<<EOF
+{% apply markdown_to_html %}
+
+Paragraph 1
+
+Paragraph 2
+{% endapply %}
+EOF, "<p>Paragraph 1</p>\n+<p>Paragraph 2</p>"],
+        ];
+    }
+
+    /**
+     * @dataProvider getIndentationTests
+     */
+    public function testStripsCommonIndentation(string $body, string $expected)
+    {
+        $runtime = new MarkdownRuntime(new class implements MarkdownInterface {
+            public function convert(string $body): string
+            {
+                return $body;
+            }
+        });
+
+        $this->assertSame($expected, $runtime->convert($body));
+    }
+
+    public static function getIndentationTests()
+    {
+        return [
+            'leading blank line keeps blank lines' => ["\nParagraph 1\n\nParagraph 2", "\nParagraph 1\n\nParagraph 2"],
+            'common indentation is removed' => ["\n    Hello\n    =====\n\n    Great!\n", "\nHello\n=====\n\nGreat!\n"],
+            'minimal common indentation is removed' => ["    a\n      b\n", "a\n  b\n"],
+            'indented code block before non-indented prose is preserved' => ["    Code\n\nParagraph\n", "    Code\n\nParagraph\n"],
+            'tab indentation is removed' => ["\tHello\n\tGreat!\n", "Hello\nGreat!\n"],
+            'mixed tabs and spaces are left untouched' => ["\ta\n    b\n", "\ta\n    b\n"],
+            'blank lines are ignored when computing indentation' => ["    a\n\n    b\n", "a\n\nb\n"],
         ];
     }