diff --git a/tests/Unit/PageIndexTest.php b/tests/Unit/PageIndexTest.php
index 08b85ac679..4af80dc144 100644
--- a/tests/Unit/PageIndexTest.php
+++ b/tests/Unit/PageIndexTest.php
@@ -5,6 +5,7 @@
 namespace SMF\Tests\Unit;
 
 use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 use SMF\Config;
 use SMF\PageIndex;
@@ -39,41 +40,63 @@ class PageIndexTest extends TestCase
 	 ****************/
 
 	/**
-	 * A negative start is how a caller says "no particular page was asked
-	 * for". The message index passes one for a topic whose first page is being
-	 * linked, and the page index answers by clamping the start to zero and
-	 * linking page 1 rather than marking it as the page you are on.
-	 *
-	 * fixStart() records that verdict as a side effect of clamping, and
-	 * __toString() calls it again on a start that has already been clamped, so
-	 * the second answer is always "valid" and the first was thrown away. Page 1
-	 * came out as plain text with no link on it, and a "next page" link
-	 * appeared beside it pointing at page 2.
+	 * Negative starts are invalid, but are clamped to zero. The invalid state
+	 * is retained so that page 1 is rendered as a link rather than as the
+	 * current page.
 	 */
+	#[DataProvider('pageIndexProvider')]
+	public function testStartIsNormalised(int $start, int $num_items, int $num_per_page, int $expected_start, int $expected_page): void
+	{
+		$page_index = new PageIndex('querystring', $start, $num_items, $num_per_page);
+
+		$this->assertSame($expected_start, $start);
+		$this->assertSame($expected_start, $page_index->start);
+		$this->assertSame($expected_page, Utils::$context['current_page']);
+
+		// Force __toString() to exercise the second fixStart() call as well.
+		(string) $page_index;
+
+		$this->assertSame($expected_start, $start);
+		$this->assertSame($expected_start, $page_index->start);
+	}
+
+	/**
+	 * The rendered current page is one-based, while current_page in the
+	 * context is zero-based.
+	 */
+	#[DataProvider('pageRenderingProvider')]
+	public function testCurrentPageIsRendered(int $start, int $num_items, int $num_per_page, int $expected_page): void
+	{
+		$page_index = new PageIndex('querystring', $start, $num_items, $num_per_page);
+
+		$this->assertSame($expected_page - 1, Utils::$context['current_page']);
+
+		$this->assertStringContainsString(\sprintf('<span class="current_page">%d</span>', $expected_page), (string) $page_index);
+	}
+
 	public function testANegativeStartLinksTheFirstPageInsteadOfMarkingIt(): void
 	{
 		$start = -1;
 		$page_index = new PageIndex('https://example.com/index.php?board=1.0', $start, 100, 20);
 
-		$this->assertStringContainsString(
-			'<a class="nav_page" href="https://example.com/index.php?board=1.0;start=0">1</a>',
-			(string) $page_index,
-		);
+		$page_index = (string) $page_index;
+
+		$this->assertSame(0, $start);
+
+		$this->assertStringContainsString('<a class="nav_page" href="https://example.com/index.php?board=1.0;start=0">1</a>', $page_index);
 
-		$this->assertStringNotContainsString('current_page', (string) $page_index);
+		$this->assertStringNotContainsString('current_page', $page_index);
 	}
 
-	/**
-	 * Nothing was navigated away from, so there is nowhere to go back to and
-	 * nothing to go on to.
-	 */
 	public function testANegativeStartShowsNeitherPreviousNorNextLinks(): void
 	{
 		$start = -1;
 		$page_index = new PageIndex('https://example.com/index.php?board=1.0', $start, 100, 20);
 
-		$this->assertStringNotContainsString('previous_page', (string) $page_index);
-		$this->assertStringNotContainsString('next_page', (string) $page_index);
+		$page_index = (string) $page_index;
+
+		$this->assertStringNotContainsString('previous_page', $page_index);
+		$this->assertStringNotContainsString('next_page', $page_index);
 	}
 
 	/**
@@ -92,6 +115,7 @@ public function testAskingTwiceGivesTheSameAnswer(): void
 	public function testTheStartValueIsClampedAndHandedBackToTheCaller(): void
 	{
 		$start = -1;
+
 		new PageIndex('https://example.com/index.php?board=1.0', $start, 100, 20);
 
 		$this->assertSame(0, $start);
@@ -106,9 +130,12 @@ public function testAStartThatNamesAPageMarksThatPageAsCurrent(): void
 		$start = 40;
 		$page_index = new PageIndex('https://example.com/index.php?board=1.0', $start, 100, 20);
 
-		$this->assertStringContainsString('<span class="current_page">3</span>', (string) $page_index);
-		$this->assertStringContainsString('previous_page', (string) $page_index);
-		$this->assertStringContainsString('next_page', (string) $page_index);
+		$page_index = (string) $page_index;
+
+		$this->assertSame(2, Utils::$context['current_page']);
+		$this->assertStringContainsString('<span class="current_page">3</span>', $page_index);
+		$this->assertStringContainsString('previous_page', $page_index);
+		$this->assertStringContainsString('next_page', $page_index);
 	}
 
 	/**
@@ -121,6 +148,9 @@ public function testAStartInTheMiddleOfAPageIsMovedToItsStart(): void
 		$page_index = new PageIndex('https://example.com/index.php?board=1.0', $start, 100, 20);
 
 		$this->assertSame(40, $start);
+		$this->assertSame(40, $page_index->start);
+		$this->assertSame(2, Utils::$context['current_page']);
+
 		$this->assertStringContainsString('<span class="current_page">3</span>', (string) $page_index);
 	}
 
@@ -134,8 +164,189 @@ public function testAStartPastTheEndLandsOnTheLastPage(): void
 		$page_index = new PageIndex('https://example.com/index.php?board=1.0', $start, 100, 20);
 
 		$this->assertSame(80, $start);
-		$this->assertStringContainsString('<span class="current_page">5</span>', (string) $page_index);
-		$this->assertStringNotContainsString('next_page', (string) $page_index);
+		$this->assertSame(80, $page_index->start);
+		$this->assertSame(4, Utils::$context['current_page']);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringContainsString('<span class="current_page">5</span>', $page_index);
+		$this->assertStringNotContainsString('next_page', $page_index);
+	}
+
+	public function testShortFormatUsesOffsetInTheUrl(): void
+	{
+		$start = 20;
+		$page_index = new PageIndex('index.php?board=1.%1$d', $start, 100, 20, true);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringContainsString('href="index.php?board=1.0"', $page_index);
+		$this->assertStringContainsString('href="index.php?board=1.80">5</a>', $page_index);
+	}
+
+	public function testDefaultFormatUsesStartParameterInTheUrl(): void
+	{
+		$start = 20;
+		$page_index = new PageIndex('index.php?board=1', $start, 100, 20);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringContainsString('href="index.php?board=1;start=0"', $page_index);
+
+		$this->assertStringContainsString('href="index.php?board=1;start=40"', $page_index);
+	}
+
+	public function testPreviousAndNextLinksCanBeDisabled(): void
+	{
+		$start = 40;
+		$page_index = new PageIndex('index.php?board=1', $start, 100, 20, false, false);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringNotContainsString('previous_page', $page_index);
+		$this->assertStringNotContainsString('next_page', $page_index);
+		$this->assertStringContainsString('<span class="current_page">3</span>', $page_index);
+	}
+
+	public function testTemplateOverridesReplaceDefaultTemplates(): void
+	{
+		$start = 20;
+
+		$page_index = new PageIndex('index.php?board=1', $start, 100, 20, false, true, [
+			'current_page' => '<strong>%1$d</strong> ',
+			'page' => '<span data-page="{URL}">%2$s</span> ',
+			'previous_page' => 'PREVIOUS ',
+			'next_page' => 'NEXT ',
+		]);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringContainsString('<strong>2</strong>', $page_index);
+		$this->assertStringContainsString('<span data-page="index.php?board=1;start=0">1</span>', $page_index);
+		$this->assertStringContainsString('PREVIOUS', $page_index);
+		$this->assertStringContainsString('NEXT', $page_index);
+	}
+
+	public function testUnknownTemplateOverrideIsIgnored(): void
+	{
+		$start = 0;
+
+		$page_index = new PageIndex('index.php?board=1', $start, 40, 20, false, true, [
+			'does_not_exist' => 'unexpected',
+		]);
+
+		$this->assertStringNotContainsString('unexpected', (string) $page_index);
+	}
+
+	public function testSetTemplateOverridesCanBeAppliedAfterConstruction(): void
+	{
+		$start = 20;
+
+		$page_index = new PageIndex('index.php?board=1', $start, 100, 20);
+
+		$page_index->setTemplateOverrides([
+			'current_page' => '<b>%1$d</b> ',
+			'previous_page' => 'BACK ',
+			'next_page' => 'FORWARD ',
+		]);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringContainsString('<b>2</b>', $page_index);
+		$this->assertStringContainsString('BACK', $page_index);
+		$this->assertStringContainsString('FORWARD', $page_index);
+	}
+
+	public function testCompactPagesCanBeDisabled(): void
+	{
+		Config::$modSettings['compactTopicPagesEnable'] = 0;
+
+		$start = 40;
+		$page_index = new PageIndex('index.php?board=1', $start, 300, 20);
+
+		$page_index = (string) $page_index;
+
+		for ($page = 1; $page <= 15; $page++) {
+			$this->assertMatchesRegularExpression(\sprintf('/>%d<\/(?:span|a)>/', $page), $page_index);
+		}
+
+		$this->assertStringNotContainsString('expandPages', $page_index);
+	}
+
+	public function testCompactPagesCanBeEnabled(): void
+	{
+		Config::$modSettings['compactTopicPagesEnable'] = 1;
+		Config::$modSettings['compactTopicPagesContiguous'] = 5;
+
+		$start = 140;
+		$page_index = new PageIndex('index.php?board=1', $start, 300, 20);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringContainsString('<span class="current_page">8</span>', $page_index);
+
+		$this->assertStringContainsString('expandPages', $page_index);
+	}
+
+	public function testOddCompactPageCountIsRoundedDown(): void
+	{
+		Config::$modSettings['compactTopicPagesEnable'] = 1;
+		Config::$modSettings['compactTopicPagesContiguous'] = 4;
+
+		$start = 140;
+		$page_index = new PageIndex('index.php?board=1', $start, 300, 20);
+
+		$page_index = (string) $page_index;
+
+		$this->assertStringContainsString('<span class="current_page">8</span>', $page_index);
+	}
+
+	/***********************
+	 * Public static methods
+	 ***********************/
+
+	/**
+	 * Provides values that exercise start normalisation.
+	 *
+	 * @return array<string, array{int, int, int, int, int}>
+	 */
+	public static function pageIndexProvider(): array
+	{
+		return [
+			'negative two' => [-2, 15, 5, 0, 0],
+			'negative one' => [-1, 15, 5, 0, 0],
+			'zero' => [0, 15, 5, 0, 0],
+
+			'first page' => [1, 15, 5, 0, 0],
+			'page two' => [5, 15, 5, 5, 1],
+			'page two through four' => [6, 15, 5, 5, 1],
+			'page three' => [10, 15, 5, 10, 2],
+
+			'past last page' => [15, 15, 5, 10, 2],
+			'far past last page' => [21, 15, 5, 10, 2],
+
+			'large values' => [3000001, 4205, 42, 4200, 100],
+
+			'max sixteen' => [6, 16, 5, 5, 1],
+			'max seventeen' => [6, 17, 5, 5, 1],
+			'max eighteen' => [6, 18, 5, 5, 1],
+			'max nineteen' => [6, 19, 5, 5, 1],
+		];
+	}
+
+	/**
+	 * Provides starts that should identify a particular rendered page.
+	 *
+	 * @return array<string, array{int, int, int, int}>
+	 */
+	public static function pageRenderingProvider(): array
+	{
+		return [
+			'first page' => [0, 100, 20, 1],
+			'second page' => [20, 100, 20, 2],
+			'middle page' => [40, 100, 20, 3],
+			'last page' => [80, 100, 20, 5],
+		];
 	}
 
 	/******************
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index a97e864c99..6b232ec369 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -64,3 +64,4 @@
 SMF\Config::$languagesdir = SMF\Config::$boarddir . '/Languages';
 SMF\Config::$cachedir = SMF\Config::$boarddir . '/cache';
 SMF\Config::$language = 'en_US';
+SMF\Config::$scripturl = 'test.local/index.php';