fix($theme-default): make dropdown-title's UI consistent with nav-link

[haoranpb]

Summary

#1837 convert dropdown-title from a to button, which brings inconsistent ui between nav-link and dropdown-title.(Especially on the phone)

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

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[haoranpb]

Hi,

While testing on Firefox, I just realize that I break the accessibility by adding outline none. The blue outline can not be shown while focusing on the dropdown-link now.

However, I think it's kind ugly without it. What do you think?

There are solutions like: remove focus immediately after click on the button. But I don't think it's very great.

I have an idea but don't have time to try it out yet:

Use color instead of the outline to indicate which title you are selecting. I mean, when you use tab to nav to a title, instead of showing an outline, change the title's color somehow.

Do you think it's practical?

[kefranabg]

Hi @ludanxer ,

Thanks for this PR, outline is important for accessibility, we have to keep it ;)

[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! 💚