New feature: Highlight popular countries in country select

app/code/core/Mage/Checkout/Block/Onepage/Abstract.php

@@ -160,22 +160,12 @@ public function getAddressesHtmlSelect($type)
      */
     public function getCountryHtmlSelect($type)
     {
-        $countryId = $this->getAddress()->getCountryId();
-        if (is_null($countryId)) {
-            $countryId = Mage::helper('core')->getDefaultCountry();
-        }
-        $select = $this->getLayout()->createBlock('core/html_select')
-            ->setName($type . '[country_id]')
-            ->setId($type . ':country_id')
-            ->setTitle(Mage::helper('checkout')->__('Country'))
-            ->setClass('validate-select')
-            ->setValue($countryId)
-            ->setOptions($this->getCountryOptions());
-        if ($type === 'shipping') {
-            $select->setExtraParams('onchange="if(window.shipping)shipping.setSameAsBilling(false);"');
-        }
-
-        return $select->getHtml();
+        return Mage::getBlockSingleton('directory/data')->getCountryHtmlSelect(
+            $this->getAddress()->getCountryId(),
+            $type . '[country_id]',
+            $type . ':country_id',
+            $this->helper('checkout')->__('Country'),
+        );
     }
 
     /**
@@ -196,6 +186,7 @@ public function getRegionHtmlSelect($type)
     }
 
     /**
+     * @deprecated
      * @return bool|mixed
      * @throws Mage_Core_Model_Store_Exception
      */
@@ -239,5 +230,4 @@ public function isShow()
     {
         return true;
     }
-    /* */
 }

app/code/core/Mage/Directory/Block/Data.php

@@ -18,6 +18,7 @@
  *
  * @category   Mage
  * @package    Mage_Directory
+ * @phpstan-type Option array{label: string, value: non-falsy-string}
  *
  * @method int getRegionId()
  */
@@ -75,7 +76,7 @@ public function getCountryHtmlSelect($defValue = null, $name = 'country_id', $id
             ->setTitle(Mage::helper('directory')->__($title))
             ->setClass('validate-select')
             ->setValue($defValue)
-            ->setOptions($options)
+            ->setOptions($this->sortCountryOptions($options))
             ->getHtml();
 
         Varien_Profiler::stop('TEST: ' . __METHOD__);
@@ -169,4 +170,29 @@ public function getRegionsJs()
         Varien_Profiler::stop('TEST: ' . __METHOD__);
         return $regionsJs;
     }
+
+    /**
+     * @template T of Option[]
+     * @param T $countryOptions
+     * @return array{0: array{label: string, value: Option[]}, 1: array{label: string, value: Option[]}}|T
+     */
+    private function sortCountryOptions(array $countryOptions): array
+    {
+        $topCountryCodes = $this->helper('directory')->getTopCountryCodes();
+        $headOptions = $tailOptions = [];
+
+        foreach ($countryOptions as $countryOption) {
+            if (in_array($countryOption['value'], $topCountryCodes)) {
+                $headOptions[] = $countryOption;
+            } else {
+                $tailOptions[] = $countryOption;
+            }
+        }
+
+        if (empty($headOptions)) {
+            return $countryOptions;
+        }
+
+        return [['label' => $this->__('Popular'), 'value' => $headOptions], ['label' => $this->__('Others'), 'value' => $tailOptions]];
+    }
 }

app/code/core/Mage/Directory/Helper/Data.php

@@ -307,4 +307,16 @@ public function isRegionRequired($countryId)
         }
         return in_array($countryId, $countyList);
     }
+
+    /** @return list<string> */
+    public function getTopCountryCodes(): array
+    {
+        $topCountries = array_filter(explode(',', (string)Mage::getStoreConfig('general/country/top_countries')));
+
+        $transportObject = new Varien_Object();
+        $transportObject->setData('top_countries', $topCountries);
+        Mage::dispatchEvent('directory_get_top_countries', ['topCountries' => $transportObject]);
+
+        return $transportObject->getData('top_countries');
+    }
 }

app/code/core/Mage/Directory/etc/system.xml

@@ -279,6 +279,16 @@
                             <show_in_store>0</show_in_store>
                             <can_be_empty>1</can_be_empty>
                         </optional_zip_countries>
+                        <top_countries translate="label">
+                            <label>Top Countries</label>
+                            <frontend_type>multiselect</frontend_type>
+                            <sort_order>40</sort_order>
+                            <source_model>adminhtml/system_config_source_country</source_model>
+                            <show_in_default>1</show_in_default>
+                            <show_in_website>1</show_in_website>
+                            <show_in_store>1</show_in_store>
+                            <can_be_empty>1</can_be_empty>
+                        </top_countries>
                     </fields>
                 </country>
                 <region translate="label">

skin/frontend/base/default/js/opcheckout.js

@@ -408,6 +408,7 @@ Shipping.prototype = {
         this.form = form;
         if ($(this.form)) {
             $(this.form).observe('submit', function(event){this.save();Event.stop(event);}.bind(this));
+            $(this.form).select('#shipping\\:country_id').first()?.addEventListener('change', () => { if (window.shipping) shipping.setSameAsBilling(false) });
         }
         this.addressUrl = addressUrl;
         this.saveUrl = saveUrl;