⏫ Forwardport of #13030 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/13030.patch (created by @chiragp-wagento) based on commit(s):
  1. 0b69c1a
  2. 7783300
  3. 78f8894
  4. 1ac8ab2

app/code/Magento/Checkout/view/frontend/templates/cart/coupon.phtml

@@ -24,7 +24,7 @@
                 <div class="field">
                     <label for="coupon_code" class="label"><span><?= /* @escapeNotVerified */ __('Enter discount code') ?></span></label>
                     <div class="control">
-                        <input type="text" class="input-text" id="coupon_code" name="coupon_code" value="<?= $block->escapeHtml($block->getCouponCode()) ?>" placeholder="<?= $block->escapeHtml(__('Enter discount code')) ?>" />
+                        <input type="text" class="input-text" id="coupon_code" name="coupon_code" value="<?= $block->escapeHtml($block->getCouponCode()) ?>" placeholder="<?= $block->escapeHtml(__('Enter discount code')) ?>" <?php if (strlen($block->getCouponCode())): ?> disabled="disabled" <?php endif; ?> />
                     </div>
                 </div>
                 <div class="actions-toolbar">

app/code/Magento/SalesRule/view/frontend/web/template/payment/discount.html

@@ -27,7 +27,7 @@
                                id="discount-code"
                                name="discount_code"
                                data-validate="{'required-entry':true}"
-                               data-bind="value: couponCode, attr:{placeholder: $t('Enter discount code')} " />
+                               data-bind="value: couponCode, attr:{disabled:isApplied() , placeholder: $t('Enter discount code')} " />
                     </div>
                 </div>
             </div>