perf: Improved Menu performance and Flip bugs

[wangKBweb]

Checklist

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• Fix linting errors
• Tests have been added / updated (or snapshots)

Change information

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[vercel]

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Name	Status	Preview	Updated
varlet	✅ Ready (Inspect)	Visit Preview	Sep 4, 2022 at 7:46PM (UTC)
varlet-ui-playground	✅ Ready (Inspect)	Visit Preview	Sep 4, 2022 at 7:46PM (UTC)

[codecov-commenter]

Codecov Report

Merging #680 (cedcb66) into v2.0 (91534b8) will decrease coverage by 0.04%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##             v2.0     #680      +/-   ##
==========================================
- Coverage   79.29%   79.24%   -0.05%     
==========================================
  Files         240      240              
  Lines        4959     4949      -10     
  Branches     1007     1004       -3     
==========================================
- Hits         3932     3922      -10     
- Misses        210      214       +4     
+ Partials      817      813       -4     

Impacted Files	Coverage Δ
packages/varlet-ui/src/menu-v2/props.ts	100.00% <ø> (ø)
packages/varlet-ui/src/select/example/index.vue	64.51% <ø> (+2.01%)	⬆️
packages/varlet-ui/src/select/Select.vue	80.46% <50.00%> (+0.89%)	⬆️
packages/varlet-ui/src/menu-v2/MenuV2.vue	77.57% <60.00%> (-0.87%)	⬇️
packages/varlet-ui/src/menu/Menu.tsx	84.00% <0.00%> (-6.00%)	⬇️

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