fix($theme-default): overlap navbar dropdown menus (fix #2227)

[billyyyyy3320]

Summary

fix #2227 #2492

Also see:
#2234
#2303
#2339
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:

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

[Mister-Hope]

LGTM

[Katheesh]

thanks for fixed this issue

[ulivz]

Thanks the this PR!

BTW, I find that when a PR fixes multiple issues, it is better for the contributors to write some background to describe it, otherwise the CR people will need to spend a lot of time to digest and understand, maybe it could be part of the contribution requirement. // cc @bencodezen

[kefranabg]

Nice!

[bencodezen]

@billyyyyy3320 Do you want to address the comment from @ulivz before merging?

[JoeyBling]

I upgraded to the latest version and there was an error,Is there a problem with local dependence?

Failed to @extend ".dropdown-title"

Error: F:\workSpaces\source-code\ibooks\node_modules@vuepress\theme-default\components\DropdownLink.vue:156:15

[bencodezen]

@JoeyBling Can you try deleting your node_modules folder and reinstalling the dependencies? If it's still a problem, please be sure to create a new issue so we can help look into it!

[JoeyBling]

@bencodezen Re installing dependencies is ok

The steps to reproduce the problem ... #2587 (comment)