fix: DH-21367: Fix shifting header group text

[vbabich]

Fix header group calculation when child columns are scrolled out of the viewport. Fall back to calculated value before the theme default column width.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 45.97%. Comparing base (7b2a23e) to head (aedbba3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/grid/src/GridRenderer.ts	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2603      +/-   ##
==========================================
+ Coverage   45.71%   45.97%   +0.26%     
==========================================
  Files         771      771              
  Lines       43649    43650       +1     
  Branches    11046    11230     +184     
==========================================
+ Hits        19954    20069     +115     
+ Misses      23678    23536     -142     
- Partials       17       45      +28     

Flag	Coverage Δ
unit	45.97% <50.00%> (+0.26%)	⬆️

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[bmingles]

LGTM