fix(comp:tree-select): panel shakes after scrolling at bottom

[sallerli1]

PR Checklist

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• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added/updated or not needed
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What is the current behavior?

tree-select在开了虚拟滚动时，在滚动到底部会导致面板内抖动

What is the new behavior?

修复以上问题

Other information

修改默认的virtualItemHeight为32

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.70%. Comparing base (403b2ca) to head (b13e133). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1942 +/- ## ========================================== - Coverage 90.71% 90.70% -0.02% ========================================== Files 360 360 Lines 35595 35595 Branches 2843 2841 -2 ========================================== - Hits 32291 32287 -4 - Misses 3304 3308 +4 ```

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