fix($theme-default): close dropdown-links when focusout on the last item (close #1948)

[haoranpb]

Summary

Fix #1948

Logic is a little complicated here. Someone please do test this again.

When people use tab to focusout the last item, the dropdown-links should be closed since the open class is surely active.

When people use mouse to focusout the last item, however, the open class may not be active.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• Related documents have been updated
• A convincing reason for adding this feature
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[kefranabg]

Hi @ludanxer,

Thanks for your work.

What you did fixed the bug, however I think that it was hard to understand (return this.open && last(array) === item). I made an improvement in the last commit. What do you think?

[haoranpb]

You make it way more readable!

[vue-bot]

Hey @ludanxer, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚