fix(ramps): cp-7.59.0 buildquote screen flickering (#22326)

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## **Description**
There was a flickering issue when changing the payment method on the buy
screen or the region on the sell screen.
To fix the payment method issue, we just needed to remove it from the
handleRegionChange dependency array.
To fix the region issue, we needed to store a reference to the
previously selected region and use that to check whether an update was
necessary.

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## **Changelog**

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`CHANGELOG entry: Added a new tab for users to see their NFTs`
`CHANGELOG entry: Fixed a bug that was causing some NFTs to flicker`

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CHANGELOG entry: null

## **Related issues**

Fixes: #22313

## **Manual testing steps**

```gherkin
Feature: Buy flow - Change payment method

  Scenario: User changes the payment method during the Buy flow without UI flickering
    Given the user is on the asset details screen
      And a default payment method is selected

    When the user taps the "Buy" button
      And the bottom sheet appears
      And the user taps "Buy" again in the bottom sheet
      And the user taps "Payment Method"
      And the user selects a different payment method

    Then the selected payment method should update
      And the UI should transition smoothly with no flickering

Feature: Sell flow - Change region

  Scenario: User changes the region during the Sell flow without UI flickering
    Given the user is on the asset details screen
      And a default region is selected

    When the user taps the "Buy" button
      And the bottom sheet appears
      And the user taps "Sell" in the bottom sheet
      And the user taps "Region"
      And the user selects a different region

    Then the selected region should update
      And the UI should transition smoothly with no flickering

```

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- [screenshots/recordings] -->

payment method change issue:

https://github.com/user-attachments/assets/53af19a7-72d3-41c3-968d-703949572049

region change issue:

https://github.com/user-attachments/assets/cf85e687-a9ea-4606-a3e9-efbced95dc42

### **After**

<!-- [screenshots/recordings] -->

payment method change fix:

https://github.com/user-attachments/assets/2635c9fd-05dc-4a1f-83e2-c35723799290

region change fix:

https://github.com/user-attachments/assets/4b103f2e-b6fc-4517-af21-6888c0b6995f


## **Pre-merge author checklist**

- [x] I’ve followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I’ve included tests if applicable
- [x] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Moves fiat-currency-on-region-change logic from BuildQuote to
useFiatCurrencies with previous-region checks, and adds tests to
validate behavior.
> 
> - **Aggregator logic**:
> - **`useFiatCurrencies`**: Adds `usePrevious(selectedRegion)` and
effect to update `selectedFiatCurrencyId` when region changes only if
using the default currency; queries new region default and updates
selection.
> - Maintains existing behaviors for selecting default currency and
validating availability.
> - **UI**:
> - **`BuildQuote.tsx`**: Removes region-change fiat update effect and
`setSelectedFiatCurrencyId` usage; relies on `useFiatCurrencies` for
currency sync.
> - **Tests**:
> - **`useFiatCurrencies.test.ts`**: Adds tests for updating currency on
region change when using default currency, and not updating when a
custom currency is selected; includes `act` usage.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
e902222. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: AxelGes

app/components/UI/Ramp/Aggregator/Views/BuildQuote/BuildQuote.tsx

@@ -141,7 +141,6 @@ const BuildQuote = () => {
     selectedRegion,
     selectedAsset,
     selectedFiatCurrencyId,
-    setSelectedFiatCurrencyId,
     selectedAddress,
     selectedNetworkName,
     sdkError,
@@ -185,7 +184,6 @@ const BuildQuote = () => {
   }, [paymentMethods, themeAppearance]);
 
   const {
-    defaultFiatCurrency,
     queryDefaultFiatCurrency,
     fiatCurrencies,
     queryGetFiatCurrencies,
@@ -247,31 +245,6 @@ const BuildQuote = () => {
     }, [shouldShowUnsupportedModal, navigation, regions, selectedRegion]),
   );
 
-  useEffect(() => {
-    const handleRegionChange = async () => {
-      if (
-        selectedRegion &&
-        selectedFiatCurrencyId === defaultFiatCurrency?.id
-      ) {
-        const newRegionCurrency = await queryDefaultFiatCurrency(
-          selectedRegion.id,
-        );
-        if (newRegionCurrency?.id) {
-          setSelectedFiatCurrencyId(newRegionCurrency.id);
-        }
-      }
-    };
-
-    handleRegionChange();
-  }, [
-    selectedRegion,
-    selectedFiatCurrencyId,
-    defaultFiatCurrency?.id,
-    queryDefaultFiatCurrency,
-    selectedPaymentMethodId,
-    setSelectedFiatCurrencyId,
-  ]);
-
   const gasLimitEstimation = useERC20GasLimitEstimation({
     tokenAddress: selectedAsset?.address,
     fromAddress: selectedAddress,

app/components/UI/Ramp/Aggregator/hooks/useFiatCurrencies.test.ts

@@ -1,3 +1,4 @@
+import { act } from '@testing-library/react-native';
 import { RampSDK } from '../sdk';
 import useFiatCurrencies from './useFiatCurrencies';
 import useSDKMethod from './useSDKMethod';
@@ -325,4 +326,107 @@ describe('useFiatCurrencies', () => {
       mockUseRampSDKValues.setSelectedFiatCurrencyId,
     ).not.toHaveBeenCalled();
   });
+
+  it('updates currency when region changes and using default currency', async () => {
+    const mockQueryDefaultFiatCurrency = jest.fn();
+    const mockSetSelectedFiatCurrencyId = jest.fn();
+
+    mockUseRampSDKValues.selectedRegion = { id: 'region-1' };
+    mockUseRampSDKValues.selectedFiatCurrencyId = 'default-fiat-currency-id';
+    mockUseRampSDKValues.setSelectedFiatCurrencyId =
+      mockSetSelectedFiatCurrencyId;
+
+    mockQueryDefaultFiatCurrency.mockResolvedValue({
+      id: 'new-region-currency',
+    });
+
+    (useSDKMethod as jest.Mock)
+      .mockReturnValueOnce([
+        {
+          data: { id: 'default-fiat-currency-id' },
+          error: null,
+          isFetching: false,
+        },
+        mockQueryDefaultFiatCurrency,
+      ])
+      .mockReturnValueOnce([
+        {
+          data: [{ id: 'default-fiat-currency-id' }],
+          error: null,
+          isFetching: false,
+        },
+        jest.fn(),
+      ])
+      .mockReturnValueOnce([
+        {
+          data: { id: 'default-fiat-currency-id' },
+          error: null,
+          isFetching: false,
+        },
+        mockQueryDefaultFiatCurrency,
+      ])
+      .mockReturnValueOnce([
+        {
+          data: [{ id: 'default-fiat-currency-id' }],
+          error: null,
+          isFetching: false,
+        },
+        jest.fn(),
+      ]);
+
+    const { rerender } = renderHookWithProvider(() => useFiatCurrencies());
+
+    mockUseRampSDKValues.selectedRegion = { id: 'region-2' };
+
+    await act(async () => {
+      rerender(() => useFiatCurrencies());
+      await new Promise((resolve) => setTimeout(resolve, 50));
+    });
+
+    expect(mockQueryDefaultFiatCurrency).toHaveBeenCalledWith('region-2');
+    expect(mockSetSelectedFiatCurrencyId).toHaveBeenCalledWith(
+      'new-region-currency',
+    );
+  });
+
+  it('does not update currency when region changes but not using default currency', async () => {
+    const mockQueryDefaultFiatCurrency = jest.fn();
+    const mockSetSelectedFiatCurrencyId = jest.fn();
+
+    mockUseRampSDKValues.selectedRegion = { id: 'region-1' };
+    mockUseRampSDKValues.selectedFiatCurrencyId = 'custom-currency-id';
+    mockUseRampSDKValues.setSelectedFiatCurrencyId =
+      mockSetSelectedFiatCurrencyId;
+
+    (useSDKMethod as jest.Mock)
+      .mockReturnValue([
+        {
+          data: { id: 'default-fiat-currency-id' },
+          error: null,
+          isFetching: false,
+        },
+        mockQueryDefaultFiatCurrency,
+      ])
+      .mockReturnValue([
+        {
+          data: [
+            { id: 'custom-currency-id' },
+            { id: 'default-fiat-currency-id' },
+          ],
+          error: null,
+          isFetching: false,
+        },
+        jest.fn(),
+      ]);
+
+    const { rerender } = renderHookWithProvider(() => useFiatCurrencies());
+
+    mockUseRampSDKValues.selectedRegion = { id: 'region-2' };
+
+    rerender(() => useFiatCurrencies());
+
+    await new Promise((resolve) => setTimeout(resolve, 50));
+
+    expect(mockSetSelectedFiatCurrencyId).not.toHaveBeenCalled();
+  });
 });

app/components/UI/Ramp/Aggregator/hooks/useFiatCurrencies.ts

@@ -1,6 +1,7 @@
 import { useEffect, useMemo } from 'react';
 import { useRampSDK } from '../sdk';
 import useSDKMethod from './useSDKMethod';
+import usePrevious from '../../../../hooks/usePrevious';
 
 export default function useFiatCurrencies() {
   const {
@@ -75,6 +76,35 @@ export default function useFiatCurrencies() {
     setSelectedFiatCurrencyId,
   ]);
 
+  const previousRegion = usePrevious(selectedRegion);
+
+  /**
+   * Update fiat currency when region changes and using default currency.
+   */
+  useEffect(() => {
+    const handleRegionChange = async () => {
+      if (selectedRegion && previousRegion?.id !== selectedRegion.id) {
+        if (selectedFiatCurrencyId === defaultFiatCurrency?.id) {
+          const newRegionCurrency = await queryDefaultFiatCurrency(
+            selectedRegion.id,
+          );
+          if (newRegionCurrency?.id) {
+            setSelectedFiatCurrencyId(newRegionCurrency.id);
+          }
+        }
+      }
+    };
+
+    handleRegionChange();
+  }, [
+    selectedRegion,
+    previousRegion,
+    selectedFiatCurrencyId,
+    defaultFiatCurrency?.id,
+    queryDefaultFiatCurrency,
+    setSelectedFiatCurrencyId,
+  ]);
+
   /**
    * Get the fiat currency object by id
    */