Fix #20930 - Prevent Firefox start failure by awaiting startUISync event

[darkwing]

Description

This PR further highlighted the problem that sometimes the UI loads before the background (most notably in Firefox). This leaves MetaMask in a state where the user sees the MetaMask fox logo and spinner but nothing actually happens. I've seen this quite a bit specifically in E2E tests.

Manual testing steps

_1. Complete usual wallet creation and subsequent login and logout, refreshing the page, etc. Everything should work as normal.

2. Checkout UX: Load the extension HTML pages and background with async JavaScript #20843
3. Run yarn build test
4. Run yarn test:e2e:single test/e2e/metamask-ui.spec.js --browser firefox --leave-running --debug
5. All tests should pass and MetaMask should properly function

Related issues

_Fixes #20930

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained:
  • What problem this PR is solving.
  • How this problem was solved.
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• I’ve indicated what issue this PR is linked to: Fixes #???
• I’ve included tests if applicable.
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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.

[github-actions]

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[Gudahtt]

Could you update the comment on this line to remove the "in MV3" part?

metamask-extension/app/scripts/metamask-controller.js

Line 3215 in baf0599

// Message startUISync is used in MV3 to start syncing state with UI

As of this change, this will be relied on in all builds. That was the only comment I could find that needs updating.

[Gudahtt]

LGTM, great work!

[brad-decker]

LGTM. Tested locally per instructions

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (a0349a6) 68.40% compared to head (8432d88) 68.40%.
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #20992   +/-   ##
========================================
  Coverage    68.40%   68.40%           
========================================
  Files         1006     1006           
  Lines        40217    40217           
  Branches     10758    10758           
========================================
  Hits         27510    27510           
  Misses       12707    12707           

Files Changed	Coverage Δ
app/scripts/metamask-controller.js	54.04% <ø> (ø)

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[metamaskbot]

Builds ready [8432d88]

• builds: chrome, firefox
• builds (beta): chrome
• builds (flask): chrome, firefox
• builds (MMI): chrome, firefox
• build viz: Build System
• mv3: Background Module Init Stats
• mv3: UI Init Stats
• mv3: Module Load Stats
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• code coverage: Report
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4
  • common: 0, 1, 2, 3, 4, 5, 6
  • content-script: 0
  • contentscript: contentscript
  • disable-console: disable-console
  • inpage: inpage
  • policy-load: policy-load
  • sentry-install: sentry-install
  • ui: 0, 1, 10, 11, 12, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (1584 ± 50 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	120	192	145	19	9
		domContentLoaded	1440	1792	1584	105	50
		load	1440	1793	1584	105	50
		domInteractive	1440	1792	1584	105	50

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: 158 Bytes (0.00%)
• common: 0 Bytes (0.00%)