gpui: Fix scroll_to_reveal_item scrolling for visible items

[Skxxtz]

Improved List Navigation: Nearest Scroll Strategy

Description

This PR refactors scroll_to_reveal_item in gpui to prevent unnecessary "jumping" when navigating through a list.

Previously, moving "Up" would aggressively snap the selected item to the top of the viewport even if it was already fully visible.

Previous behavior:

2026-03-27.11-43-34.mp4

New behavior:

2026-03-27.11-45-25.mp4

How:

The new implementation uses pixel-coordinate math (via SumTree cursors) to implement a Nearest scroll strategy:

• If the item is already fully visible: The scroll position remains unchanged.
• If the item is above the viewport: The list scrolls just enough to align the item's top with the viewport's top.
• If the item is below the viewport: The list scrolls just enough to align the item's bottom with the viewport's bottom.

This creates a much smoother experience in the Launcher and other list-heavy interfaces where users frequently toggle between adjacent items.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tested locally on a project
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed "jumpy" list scrolling by ensuring items only trigger a scroll when they are actually outside the visible area.