Edits for bug magento/magento2/#11998

Author: Jack Krielen

app/code/Magento/Catalog/Block/Product/View.php

@@ -196,6 +196,10 @@ public function getJsonConfig()
             'productId'   => (int)$product->getId(),
             'priceFormat' => $this->_localeFormat->getPriceFormat(),
             'prices'      => [
+                'baseOldPrice' => [
+                    'amount'      => $priceInfo->getPrice('regular_price')->getAmount()->getBaseAmount() * 1,
+                    'adjustments' => []
+                ],
                 'oldPrice'   => [
                     'amount'      => $priceInfo->getPrice('regular_price')->getAmount()->getValue() * 1,
                     'adjustments' => []

app/code/Magento/ConfigurableProduct/Block/Product/View/Type/Configurable.php

@@ -303,6 +303,11 @@ protected function getOptionPrices()
 
             $prices[$product->getId()] =
                 [
+                    'baseOldPrice' => [
+                        'amount' => $this->localeFormat->getNumber(
+                            $priceInfo->getPrice('regular_price')->getAmount()->getBaseAmount()
+                        ),
+                    ],
                     'oldPrice' => [
                         'amount' => $this->localeFormat->getNumber(
                             $priceInfo->getPrice('regular_price')->getAmount()->getValue()

app/code/Magento/ConfigurableProduct/Model/Product/Type/Configurable/Variations/Prices.php

@@ -39,6 +39,9 @@ public function getFormattedPrices(\Magento\Framework\Pricing\PriceInfo\Base $pr
         $finalPrice = $priceInfo->getPrice('final_price');
 
         return [
+            'baseOldPrice' => [
+                'amount' => $this->localeFormat->getNumber($regularPrice->getAmount()->getBaseAmount()),
+            ],
             'oldPrice' => [
                 'amount' => $this->localeFormat->getNumber($regularPrice->getAmount()->getValue()),
             ],

app/code/Magento/Tax/Pricing/Render/Adjustment.php

@@ -173,4 +173,17 @@ public function displayPriceExcludingTax()
     {
         return $this->taxHelper->displayPriceExcludingTax();
     }
+
+    /**
+     * Obtain a value for data-price-type attribute
+     *
+     * @return string
+     */
+    public function getDataPriceType()
+    {
+        if ( $this->getData('price_type') !== 'finalPrice' && $priceType = $this->getData('price_type')) {
+            return 'base' . ucfirst($priceType);
+        }
+        return 'basePrice';
+    }
 }

app/code/Magento/Tax/view/base/templates/pricing/adjustment.phtml

@@ -6,12 +6,13 @@
 ?>
 
 <?php /** @var \Magento\Tax\Pricing\Render\Adjustment $block */ ?>
+<?php /** @var $escaper \Magento\Framework\Escaper */ ?>
 
 <?php if ($block->displayBothPrices()) : ?>
-    <span id="<?= $block->escapeHtmlAttr($block->buildIdWithPrefix('price-excluding-tax-')) ?>"
-          data-label="<?= $block->escapeHtmlAttr(__('Excl. Tax')) ?>"
+    <span id="<?= $escaper->escapeHtmlAttr($block->buildIdWithPrefix('price-excluding-tax-')) ?>"
+          data-label="<?= $escaper->escapeHtmlAttr(__('Excl. Tax')) ?>"
           data-price-amount="<?= /* @noEscape */ $block->getRawAmount() ?>"
-          data-price-type="basePrice"
+          data-price-type="<?= $escaper->escapeHtmlAttr($block->getDataPriceType()); ?>"
           class="price-wrapper price-excluding-tax">
         <span class="price"><?= /* @noEscape */ $block->getDisplayAmountExclTax() ?></span></span>
 <?php endif; ?>