Fix initial discount calculation on draft order line (#1651)

src/hooks/useUpdateEffect.ts

@@ -0,0 +1,14 @@
+import { useEffect, useRef } from "react";
+
+export function useUpdateEffect(fn: () => void, depArr: any[]) {
+  const mounted = useRef(false);
+
+  useEffect(() => {
+    if (mounted.current) {
+      return fn();
+    }
+    mounted.current = true;
+  }, depArr);
+}
+
+export default useUpdateEffect;

src/orders/components/OrderDiscountCommonModal/OrderDiscountCommonModal.tsx

@@ -14,6 +14,7 @@ import ConfirmButton, {
 import PriceField from "@saleor/components/PriceField";
 import RadioGroupField from "@saleor/components/RadioGroupField";
 import { Money } from "@saleor/fragments/types/Money";
+import { useUpdateEffect } from "@saleor/hooks/useUpdateEffect";
 import { buttonMessages } from "@saleor/intl";
 import { makeStyles } from "@saleor/macaw-ui";
 import { DiscountValueTypeEnum } from "@saleor/types/globalTypes";
@@ -258,7 +259,7 @@ const OrderDiscountCommonModal: React.FC<OrderDiscountCommonModalProps> = ({
     setValue(recalculatedValue);
   };
 
-  useEffect(handleValueConversion, [calculationMode]);
+  useUpdateEffect(handleValueConversion, [calculationMode]);
 
   const dialogTitle =
     modalType === ORDER_LINE_DISCOUNT