fix: rollup rows dropzone area in table sidebar

[dsmmcken]

Previous behaviour would grow the drop-zone when an item is picked up, but that expanded area would not actually be droppable. Leading to frustrating ux where it seemed like you dropped an item in the droppable area, but it would revert. The list needs to grow, so that you can place the item above or below any existing items.

Root cause is DnD doesn't treat that area as droppable unless it is overlapping the list. This change enables the DnD placeholder item, to fill out the expanded area. The list now grows after initial contact rather than before, as that's when the placeholder gets added by DnD.

Additionally, added a hover styling to positively show your drop is going to be successful. Ultimately though, this goes away entirely with pivot builder.

[Copilot]

Pull Request Overview

This PR fixes the dropzone area behavior in the rollup rows table sidebar by improving how the drop target height is calculated and displayed during drag-and-drop operations.

Key Changes:

• Replaced dynamic JavaScript-based height calculation with CSS custom properties for more reliable dropzone sizing
• Added hasPlaceholder prop to DraggableItemList to conditionally render drag-and-drop placeholders
• Enhanced visual feedback with background color changes when dragging over drop zones

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
packages/iris-grid/src/sidebar/RollupRows.tsx	Removed conditional height calculation logic and passes CSS custom properties for height management; added hasPlaceholder prop to enable placeholder rendering
packages/iris-grid/src/sidebar/RollupRows.scss	Implemented CSS custom property-based height calculation with min-height, added visual feedback for drag-over states, and improved text wrapping
packages/components/src/DraggableItemList.tsx	Added optional hasPlaceholder prop with default value false to conditionally render the droppable placeholder element

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.17%. Comparing base (e1c2dc4) to head (1c4da62).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/components/src/DraggableItemList.tsx	0.00%	1 Missing ⚠️
packages/iris-grid/src/sidebar/RollupRows.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2572      +/-   ##
==========================================
+ Coverage   44.54%   45.17%   +0.62%     
==========================================
  Files         768      769       +1     
  Lines       43367    43462      +95     
  Branches    10973    11170     +197     
==========================================
+ Hits        19317    19632     +315     
+ Misses      24034    23784     -250     
- Partials       16       46      +30     

Flag	Coverage Δ
unit	45.17% <0.00%> (+0.62%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.