fix: fix add custom assets (#8872)

## Explanation

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## References

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## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches `addCustomAsset` state writes and its follow-up `getAssets`
request parameters, which can affect token rendering and immediate
post-import fetching behavior across chains.
> 
> **Overview**
> Fixes custom-asset import UX by having `addCustomAsset` immediately
**seed `assetsBalance` with `{ amount: '0' }`** for the newly added
asset (without overwriting an existing balance), allowing the UI to
render the token before the next pipeline fetch.
> 
> Also tightens the post-import refresh by explicitly fetching
`balance`, `metadata`, and `price` for *fungible* assets on the token’s
chain, and adds regression tests + a changelog entry for the behavior.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
8c40ecf. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: salimtb

packages/assets-controller/CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Export `SnapControllerSnapInstalledEvent` from `AssetsControllerMessenger` allowed events type ([#8868](https://github.com/MetaMask/core/pull/8868))
+- `addCustomAsset` now immediately seeds `assetsBalance` with a zero amount for the newly added asset, so the UI can render it before the next pipeline fetch ([#8872](https://github.com/MetaMask/core/pull/8872))
 
 ## [8.0.0]
 

packages/assets-controller/src/AssetsController.test.ts

@@ -523,6 +523,37 @@ describe('AssetsController', () => {
         expect(controller.state.customAssets[MOCK_ACCOUNT_ID]).toHaveLength(2);
       });
     });
+
+    it('seeds assetsBalance with a zero amount for a newly added custom asset', async () => {
+      await withController(async ({ controller }) => {
+        await controller.addCustomAsset(MOCK_ACCOUNT_ID, MOCK_ASSET_ID);
+
+        expect(
+          controller.state.assetsBalance[MOCK_ACCOUNT_ID]?.[MOCK_ASSET_ID],
+        ).toStrictEqual({ amount: '0' });
+      });
+    });
+
+    it('does not overwrite an existing balance when re-adding a custom asset', async () => {
+      await withController(
+        {
+          state: {
+            assetsBalance: {
+              [MOCK_ACCOUNT_ID]: {
+                [MOCK_ASSET_ID]: { amount: '1000000' },
+              },
+            },
+          },
+        },
+        async ({ controller }) => {
+          await controller.addCustomAsset(MOCK_ACCOUNT_ID, MOCK_ASSET_ID);
+
+          expect(
+            controller.state.assetsBalance[MOCK_ACCOUNT_ID]?.[MOCK_ASSET_ID],
+          ).toStrictEqual({ amount: '1000000' });
+        },
+      );
+    });
   });
 
   describe('removeCustomAsset', () => {

packages/assets-controller/src/AssetsController.ts

@@ -1689,6 +1689,15 @@ export class AssetsController extends BaseController<
         (state.assetsInfo as Record<string, AssetMetadata>)[normalizedAssetId] =
           assetMetadata;
       }
+
+      // Seed a zero balance so the UI can render the asset immediately,
+      // before the next pipeline fetch returns the real amount.
+      const balances = state.assetsBalance as Record<
+        string,
+        Record<string, AssetBalance>
+      >;
+      balances[accountId] ??= {};
+      balances[accountId][normalizedAssetId] ??= { amount: '0' };
     });
 
     // Fetch data for the newly added custom asset (merge to preserve other chains)
@@ -1697,6 +1706,8 @@ export class AssetsController extends BaseController<
       const chainId = extractChainId(normalizedAssetId);
       await this.getAssets([account], {
         chainIds: [chainId],
+        dataTypes: ['balance', 'metadata', 'price'],
+        assetTypes: ['fungible'],
         forceUpdate: true,
         updateMode: 'merge',
       });