… with quote loader (#31218)

## **Description**

In the QuickBuy bottom sheet, the **Buy** button sometimes took longer
to become enabled than the amount-section quote loader took to
disappear. The user expects the CTA to become tappable as soon as the
quote completes.

**Reason for the change:** the loader and the Buy button were gated by
two conditions that drifted out of sync for one render. The loader is
driven by `isBlockingQuoteLoad` (gated by `isQuoteLoading`), which
clears the instant the fetch resolves. The button is gated by
`isConfirmDisabled`, which includes `isPendingQuoteRefresh` — and that
value was derived from `settledSourceTokenAmountRef`, a ref updated only
inside a post-commit `useEffect`. On the render where the quote arrived,
the loader hid but the ref had not caught up, and ref writes do not
schedule a re-render, so the CTA only re-enabled on the next unrelated
render (a redux tick, gas-estimate update, quote countdown, etc.) —
producing the variable lag.

**Solution:** replace the timing/effect-based "settled amount" tracking
with a synchronous derivation. A displayed quote is for the current
amount when the quote's own `srcTokenAmount` equals the requested amount
(the request is built with `calcTokenValue(sourceTokenAmount,
decimals).toFixed(0)` and the bridge echoes that exact value back).
`isPendingQuoteRefresh` now derives from this synchronous check, so
`isBlockingQuoteLoad` and `isConfirmDisabled` settle on the **same
render** the matching quote lands. Background refreshes of the same
amount keep `srcTokenAmount` unchanged, so the CTA stays enabled as
before.

## **Changelog**

CHANGELOG entry: Fixed the QuickBuy Buy button sometimes staying
disabled briefly after the quote finished loading.

## **Related issues**

Fixes: An issue where he QuickBuy Buy button sometimes staying disabled
briefly after the quote finished loading.

## **Manual testing steps**

```gherkin
Feature: QuickBuy Buy button enablement

  Scenario: Buy button enables as soon as the quote loads
    Given I open the QuickBuy bottom sheet for a token
    When I enter or select an amount and the quote loader spins
    Then the loader disappears when the quote completes
    And the Buy button becomes enabled on the same frame (no lag)

  Scenario: Buy button stays disabled while a new amount is being quoted
    Given a quote is already displayed for an amount
    When I change the amount so a new quote is fetched
    Then the Buy button is disabled until the quote for the new amount arrives
```

## **Screenshots/Recordings**

### **Before**

N/A

### **After**

N/A

## **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.

#### Performance checks (if applicable)

- [x] I've tested on Android
  - Ideally on a mid-range device; emulator is acceptable
- [x] I've tested with a power user scenario
- Use these [power-user
SRPs](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/edit-v2/401401446401?draftShareId=9d77e1e1-4bdc-4be1-9ebb-ccd916988d93)
to import wallets with many accounts and tokens
- [x] I've instrumented key operations with Sentry traces for production
performance metrics
- See [`trace()`](/app/util/trace.ts) for usage and
[`addToken`](/app/components/Views/AddAsset/components/AddCustomToken/AddCustomToken.tsx#L274)
for an example

For performance guidelines and tooling, see the [Performance
Guide](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/400085549067/Performance+Guide+for+Engineers).

## **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.

Made with [Cursor](https://cursor.com)


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Localized QuickBuy confirm/disable logic with added unit tests; no
auth, submission, or bridge API changes beyond when the button enables.
> 
> **Overview**
> Fixes QuickBuy **Buy** staying disabled briefly after the quote loader
finishes by replacing post-commit ref tracking of the “settled” amount
with a synchronous **`isActiveQuoteForCurrentAmount`** check.
> 
> **`isPendingQuoteRefresh`** now derives from whether the on-screen
quote matches the committed amount: atomic units from
**`calcTokenValue`** on the requested **`sourceTokenAmount`** are
compared to **`activeQuote.sentAmount`** (full wallet deduction), not
**`quote.srcTokenAmount`**, so gas-included/sponsored quotes still
enable the CTA when the displayed quote is valid.
> 
> Tests add default **`sentAmount`** on quote fixtures plus cases for
same-render loader/CTA sync and post-fee **`srcTokenAmount`** with
matching **`sentAmount`**.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
4124eb2. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Cursor <cursoragent@cursor.com>