Resolves z-index stacking context issue by using portal rendering

[dmnyc]

Summary

Fixes a z-index bug where the "Trending 24h" dropdown menu in the main timeline would appear behind image galleries in posts.

Changes

• SelectionBox2.tsx: Wrapped Select.Content with Select.Portal to render dropdown outside normal DOM hierarchy
• SelectionBox2.tsx: Added positioning props (gutter={8}, placement="bottom-start", shift={-8}, sameWidth={false}) to maintain original dropdown alignment
• SelectionBox.module.scss: Increased dropdown z-index from var(--z-index-lifted) (10) to 100
• HomeSidebar.module.scss: Increased sidebar heading z-index from 5 to 50
• Layout.module.scss: Added position: relative; z-index: 100 to .rightColumn

Root Cause

The .centerColumn element had position: relative, creating an isolated stacking context. This prevented the dropdown (rendered as a child of .rightColumn) from appearing above content in the center column, regardless of z-index values.

Solution

Using Select.Portal renders the dropdown at the document body level, completely escaping all parent stacking contexts. This ensures the dropdown appears above all page content while maintaining proper positioning relative to its trigger.

Test Plan

• Verify dropdown appears above image galleries in posts
• Verify dropdown positioning matches original alignment
• Tested with top 24-hour trending post ID:
  nevent1qvzqqqqqqypzqeqq2md08cn2vft4vd3yk49qkw6hn8t59zc7yp770mxwln3j4ak2qqsvv968dyyrl4utjc0g9zptywvzqk3wj7glnemexsr4lqwsa59xzysxj23l5

Screenshots

Before:

After:

Dev note: This fix was developed with Claude Sonnet 4.5.