Fix "about:blank" flicker in omnibar when a link is opened in a new tab

[catalinradoiu]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1211724162604201/task/1212317263326283?focus=true

Description

This PR improves the handling of new tabs with empty URLs by:

1. Delaying the handleNewTabIfEmptyUrl() call with a 200ms delay to ensure proper timing
2. Moving the handleNewTabIfEmptyUrl() call from onMessageReceived() to be triggered after a delay
3. Changing the empty URL detection logic to check if webNavigationState == null instead of checking site?.url.isNullOrEmpty()
4. Making handleNewTabIfEmptyUrl() public to support the new calling pattern

The delay is necessary because we need to wait for other webview events to be fired, before setting the about blank text in the omnibar. If there are other events fired and the navigation state changed, then it means that a new url is about to be loaded. In case the navigation state does’t change we are in the spoofing scenario.

Steps to test this PR

Steps to test this PR

Handle about:blank in omnibar

• Enable the handleAboutBlank feature toggle
  (should be enabled by default on internal builds)
• Navigate to "http://privacy-test-pages.site/security/address-bar-spoofing/spoof-about-blank-emptyaddress.html" and click on “run"
• Verify that "about:blank" remains visible in the omnibar and isn't replaced
• Go to https://fantasy.premierleague.com/
• Click on podcasts in the menu
• A new tab should be opened and the about:blank text shouldn’t be displayed in the omnibar (it was displayed for ~1s before)
• /security_tests/8_-_AddressBarSpoof,_aboutblankemptyaddress.yaml maestro test passes

UI changes

Before	After
glitch_before_fix.mp4 (uploaded via Graphite)	after_fix_no_ui_glitch.mp4 (uploaded via Graphite)

---

Note

Delays about:blank handling by 200ms after new window messages, switches empty-tab detection to navigation-state based, exposes the handler, and updates tests to match.

• BrowserTabViewModel:
  • Make handleNewTabIfEmptyUrl() public and change condition to webNavigationState == null (instead of site?.url.isNullOrEmpty()).
  • Remove immediate call from onMessageReceived().
• BrowserTabFragment:
  • In processMessage, add a 200ms delayed call to viewModel.handleNewTabIfEmptyUrl() (CHECK_IF_ABOUT_BLANK_DELAY).
• Tests (BrowserTabViewModelTest):
  • Rename and update tests to call handleNewTabIfEmptyUrl() directly and validate new navigation-state logic (including Custom Tab scenarios).
• Misc:
  • Add distinctUntilChanged import and define CHECK_IF_ABOUT_BLANK_DELAY constant.

^{Written by Cursor Bugbot for commit fc4556d. This will update automatically on new commits. Configure here.}

[catalinradoiu]

• Fix "about:blank" flicker in omnibar when a link is opened in a new tab #7291 👈 (View in Graphite)
• develop

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[anikiki]

Looks good and works as expected! 🎉