fix: DH-20229: Value is null or undefined when scrolling rollup

[ericlln]

The "Value is null or undefined" error was thrown from calling IrisGridTreeTableModel.textAlignForCellwhen scrolling a table with expanded rollups while hovering on a row.

I think these errors are due to textAlignForCell not being able to find the row in the viewport when scrolling rapidly as it has already left the viewport, and so it shouldn't matter what is returned in this case if the cell isn't displayed on screen.

• In this case, the row is only used to determine whether the constituent type should be used for formatting, and will now fallback to the column type when the row is null.

To address potential issues with scrolling in the future, some tests involving scrolling a table were added. These mainly represent the failure scenario seen in the ticket, but more can be added in the future to test other scenarios.

• From running the tests, a similar error was found in IrisGridRenderer.getExpandButtonPosition, and a fallback value has been added to replace the not null assertions there as well. This is the same pattern used in IrisGridTextCellRenderer.ts.getCellOverflowButtonPosition

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.94%. Comparing base (6938c47) to head (fba5f7e).

Files with missing lines	Patch %	Lines
packages/iris-grid/src/IrisGridRenderer.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2538      +/-   ##
==========================================
- Coverage   44.94%   44.94%   -0.01%     
==========================================
  Files         765      765              
  Lines       42915    42913       -2     
  Branches    10992    10993       +1     
==========================================
- Hits        19290    19289       -1     
+ Misses      23612    23563      -49     
- Partials       13       61      +48     

Flag	Coverage Δ
unit	44.94% <0.00%> (-0.01%)	⬇️

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.