Show right number of tabs when closing with mouse on first tab in new…

[bbondy]

… tab page

Fix #9561

Auditors: @bsclifton

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Automated test plan

npm run test -- --grep="shows the right number of tabs after closing with mouse"

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

Changes look good; test works perfectly as described. Since the test matches the steps specified so closely, I marked this as no-qa-needed. Great job 😄