Add the ability to debug / force load the JS polyfill.

fast-smooth-scroll.php

@@ -97,6 +97,23 @@ function fast_smooth_scroll_enqueue_scripts() {
 		return;
 	}
 
+	/*
+	 * Administrators can force the polyfill to load by adding a query parameter `fast_smooth_scroll_debug_polyfill=1`
+	 * to any URL. In this case, the polyfill is unconditionally enqueued.
+	 * It also overrides the default scroll behavior to 'auto' to simulate the experience without 'smooth' scrolling
+	 * configured via CSS.
+	 */
+	// phpcs:ignore WordPress.Security.NonceVerification.Recommended
+	if ( current_user_can( 'manage_options' ) && ! empty( $_GET['fast_smooth_scroll_debug_polyfill'] ) ) {
+		wp_add_inline_script(
+			'fast-smooth-scroll-scroll-behavior-polyfill',
+			'document.documentElement.style.scrollBehavior = "auto";',
+			'before'
+		);
+		wp_enqueue_script( 'fast-smooth-scroll-scroll-behavior-polyfill' );
+		return;
+	}
+
 	wp_enqueue_script( 'fast-smooth-scroll-polyfills' );
 }
 add_action( 'wp_enqueue_scripts', 'fast_smooth_scroll_enqueue_scripts' );

readme.txt

@@ -69,6 +69,16 @@ add_filter( 'fast_smooth_scroll_enqueue_scripts', '__return_false' );
 
 `
 
+= How can I test the JavaScript polyfill? =
+
+Most likely, you are using a modern browser which therefore does not trigger the JavaScript polyfill to load.
+
+If you don't have a legacy browser handy, you can still test the behavior: You'll need to be logged in as an administrator, and then you can add a query parameter `fast_smooth_scroll_debug_polyfill=1` to any URL. For example, in case of the home page:
+
+`
+https://my-site.com/?fast_smooth_scroll_debug_polyfill=1
+`
+
 = Where should I submit my support request? =
 
 For regular support requests, please use the [wordpress.org support forums](https://wordpress.org/support/plugin/fast-smooth-scroll). If you have a technical issue with the plugin where you already have more insight on how to fix it, you can also [open an issue on GitHub instead](https://github.com/felixarntz/fast-smooth-scroll/issues).

tests/phpunit/tests/Plugin_Tests.php

@@ -96,4 +96,57 @@ public function test_fast_smooth_scroll_enqueue_scripts_filter() {
 		$this->assertTrue( $loader_registered );
 		$this->assertFalse( $loader_enqueued );
 	}
+
+	public function test_fast_smooth_scroll_enqueue_scripts_with_debug_polyfill_param() {
+		global $wp_scripts;
+
+		// Store original `$wp_scripts`, then reset it.
+		$orig_wp_scripts = wp_scripts();
+		$wp_scripts      = null;
+
+		// Set the query parameter. It shouldn't change any behavior by itself.
+		$_GET['fast_smooth_scroll_debug_polyfill'] = '1';
+
+		fast_smooth_scroll_register_scripts();
+		fast_smooth_scroll_enqueue_scripts();
+
+		$polyfill_enqueued      = wp_script_is( 'fast-smooth-scroll-scroll-behavior-polyfill', 'enqueued' );
+		$loader_enqueued        = wp_script_is( 'fast-smooth-scroll-polyfills', 'enqueued' );
+		$polyfill_inline_script = wp_scripts()->get_inline_script_data( 'fast-smooth-scroll-scroll-behavior-polyfill', 'before' );
+
+		// Restore original `$wp_scripts`.
+		$wp_scripts = $orig_wp_scripts;
+
+		// Ensure that still only the loader is enqueued, as the query parameter can only be used by administrators.
+		$this->assertFalse( $polyfill_enqueued );
+		$this->assertTrue( $loader_enqueued );
+		$this->assertSame( '', $polyfill_inline_script );
+	}
+
+	public function test_fast_smooth_scroll_enqueue_scripts_with_debug_polyfill_param_and_administrator() {
+		global $wp_scripts;
+
+		// Store original `$wp_scripts`, then reset it.
+		$orig_wp_scripts = wp_scripts();
+		$wp_scripts      = null;
+
+		// Set the current user to an administrator, and set the query parameter. This should force load the polyfill.
+		wp_set_current_user( self::factory()->user->create( array( 'role' => 'administrator' ) ) );
+		$_GET['fast_smooth_scroll_debug_polyfill'] = '1';
+
+		fast_smooth_scroll_register_scripts();
+		fast_smooth_scroll_enqueue_scripts();
+
+		$polyfill_enqueued      = wp_script_is( 'fast-smooth-scroll-scroll-behavior-polyfill', 'enqueued' );
+		$loader_enqueued        = wp_script_is( 'fast-smooth-scroll-polyfills', 'enqueued' );
+		$polyfill_inline_script = wp_scripts()->get_inline_script_data( 'fast-smooth-scroll-scroll-behavior-polyfill', 'before' );
+
+		// Restore original `$wp_scripts`.
+		$wp_scripts = $orig_wp_scripts;
+
+		// Ensure that the polyfill is now force enqueued instead of the loader, and the extra inline script is added.
+		$this->assertTrue( $polyfill_enqueued );
+		$this->assertFalse( $loader_enqueued );
+		$this->assertSame( 'document.documentElement.style.scrollBehavior = "auto";', $polyfill_inline_script );
+	}
 }