Wrong assert in NodeTitleTest

So, I think the real problem here is that the H1 is not rendered at all as best I can tell.

I a possible change to the test would be:

--- a/core/modules/node/src/Tests/NodeTitleTest.php
+++ b/core/modules/node/src/Tests/NodeTitleTest.php
@@ -85,7 +85,7 @@ function testNodeTitle() {
     $this->assertTitle(0 . ' | Drupal', 'Page title is equal to 0.', 'Node');
     // Test that 0 appears in the template <h2>.
     $xpath = '//h1';
-    $this->assertEqual(current($this->xpath($xpath)), 0, 'Node title is displayed as 0.', 'Node');
+    $this->assertIdentical((string) current($this->xpath($xpath)), '0', 'Node title is displayed as 0.', 'Node');
 
     // Test edge case where node title contains special characters.
     $edge_case_title = 'article\'s "title".';

And this is showing a bug perhaps in the rendering system to node template?

// Test that 0 appears in the template <h1>.
$xpath = '//h1';
$this->assertEqual(current($this->xpath($xpath)), 0, 'Node title is displayed as 0.', 'Node');

Page title is not rendered now, so test (current($this->xpath($xpath))) treated 0 as FALSE and following assert pass! See attached test-only.

$this->assertEqual(FALSE, 0, 'Node title is displayed as 0.', 'Node');

Updated issue summary and fixed coding standards and deprecated code.

Changed assertEqual() to assertSame(). Asserts that two variables have the same type and value.

Added test-only.

Nice fix. Testing for things that PHP duck types to false can be tricky.

+++ b/core/modules/node/tests/src/Functional/NodeTitleTest.php
@@ -34,8 +34,14 @@ class NodeTitleTest extends NodeTestBase {
-    $this->adminUser = $this->drupalCreateUser(['administer nodes', 'create article content', 'create page content', 'post comments']);
...
+    $this->adminUser = $this->drupalCreateUser([
+      'administer nodes',
+      'create article content',
+      'create page content',
+      'post comments',
+    ]);

@@ -56,15 +62,16 @@ public function testNodeTitle() {
-    $this->assertEqual($this->xpath($xpath)[0]->getText(), $node->label() . ' | Drupal', 'Page title is equal to node title.', 'Node');
+    $this->assertEquals($node->label() . ' | Drupal', $this->xpath($xpath)[0]->getText(), 'Page title is equal to node title.');
...
-    $this->assertEqual($this->xpath($xpath)[0]->getText(), $node->label(), 'Node breadcrumb is equal to node title.', 'Node');
+    $this->assertEquals($node->label(), $this->xpath($xpath)[0]->getText(), 'Node breadcrumb is equal to node title.');
...
-    $this->assertEqual($this->xpath('//article[contains(concat(" ", normalize-space(@class), " "), :node-class)]/h2/a/span', [':node-class' => ' node--type-' . $node->bundle() . ' '])[0]->getText(), $node->label(), 'Node preview title is equal to node title.', 'Node');
+    $xpath = '//article[contains(concat(" ", normalize-space(@class), " "), :node-class)]/h2/a/span';
+    $this->assertEquals($node->label(), $this->xpath($xpath, [':node-class' => ' node--type-' . $node->bundle() . ' '])[0]->getText(), 'Node preview title is equal to node title.');

@@ -92,12 +98,11 @@ public function testNodeTitle() {
-    $this->assertRaw('<title>' . $edge_case_title_escaped . ' | Drupal</title>', 'Page title is equal to article\'s "title".', 'Node');
+    $this->assertSession()->responseContains('<title>' . $edge_case_title_escaped . ' | Drupal</title>', 'Page title is equal to article\'s "title".');
...
-    $this->assertRaw('<title>' . $edge_case_title_escaped . ' | Drupal</title>', 'Page title is equal to article\'s "title".', 'Node');
-
+    $this->assertSession()->responseContains('<title>' . $edge_case_title_escaped . ' | Drupal</title>', 'Page title is equal to article\'s "title".');

These changes are out-of-scope. Yes at some point we need to make this change or similar changes. But scope creep makes everyone have to think more to review the issue.

+++ b/core/modules/node/tests/src/Functional/NodeTitleTest.php
@@ -77,10 +78,10 @@ public function testNodeTitle() {
-    $this->assertTitle(0 . ' | Drupal', 'Page title is equal to 0.', 'Node');
+    $this->assertSession()->titleEquals('0 | Drupal');

This is also out of scope, it's not needed to fix the problem right?

Test only fix so should be eligible for 8.6.x

This is also out of scope, it's not needed to fix the problem right?

Right, it's not important in this issue.

 * @deprecated Scheduled for removal in Drupal 9.0.0.
 *   Use $this->assertSession()->titleEquals() instead.
 */
protected function assertTitle($expected_title) {

This looks good now, all feedback has been addressed, thanks @Krzysztof Domański

Crediting @wheatpenny for creating the issue and @Lendude and myself for scope management.

Committed and pushed 751b38424c to 8.7.x and 11f66339c1 to 8.6.x. Thanks!