Correct header rendering for joined columns

[paodb]

Close #190

CSV/DOCX/PDF were always rendering the topmost header row, which for joined headers produced the joined-cell text instead of the actual column header. They now use the header row closest to the data, honoring setCustomHeader if set. In Excel, setCustomHeader was being applied to every header row (duplicating the value across joined rows); it now applies only to the row closest to the data. A new demo covers the issue's scenario.

Summary by CodeRabbit

• New Features
  • Added support for multi-level header rows in grid exports, enabling grouped or hierarchical column headers.
  • Custom header text can now be configured on a per-column basis; these settings apply to the header row immediately adjacent to the exported data.
  • Multi-level header behavior varies across different export formats.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: faf6ad8a-d3e8-462e-a0da-fb104acee651

📥 Commits

Reviewing files that changed from the base of the PR and between cf9dd42 and 7ac9db7.

📒 Files selected for processing (4)

• src/main/java/com/flowingcode/vaadin/addons/gridexporter/BaseStreamResourceWriter.java
• src/main/java/com/flowingcode/vaadin/addons/gridexporter/GridHeader.java
• src/test/java/com/flowingcode/vaadin/addons/gridexporter/GridExporterDemoView.java
• src/test/java/com/flowingcode/vaadin/addons/gridexporter/GridExporterJoinedHeadersWithCustomHeaderDemo.java

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Walkthrough

When grids contain joined header rows, the exporter now applies custom headers only to the final header row and uses component-derived text for earlier rows. GridHeader.getText() returns the last text element. A new demo illustrates joined-header grid export with per-column custom header mapping.

Changes

Joined Header Row Custom Header Handling

Layer / File(s)	Summary
Header row iteration and custom header routing src/main/java/com/flowingcode/vaadin/addons/gridexporter/BaseStreamResourceWriter.java	getGridHeader switches to index-based iteration and tracks the last header row, passing an isLastRow flag to renderHeaderCellTextContent. That method now applies GridExporter.COLUMN_HEADER custom headers only when processing the last row; other rows receive null to trigger fallback text extraction from grid cell components.
Header text element selection src/main/java/com/flowingcode/vaadin/addons/gridexporter/GridHeader.java	getText() returns the final element of the texts list instead of the first, aligning with the last-row header preference established in the previous layer.
Demo: joined headers with custom headers src/test/java/com/flowingcode/vaadin/addons/gridexporter/GridExporterDemoView.java, src/test/java/com/flowingcode/vaadin/addons/gridexporter/GridExporterJoinedHeadersWithCustomHeaderDemo.java	New demo class GridExporterJoinedHeadersWithCustomHeaderDemo constructs a grid with two columns (First Name, Last Name), prepends a joined header row spanning both, populates sample data via Faker, and configures per-column custom headers (Given name, Family name) that apply to the bottom row. Registration is added to GridExporterDemoView.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• FlowingCode/GridExporterAddon#154: Similar header-row selection and per-row custom header routing in BaseStreamResourceWriter, with accompanying demo view updates.
• FlowingCode/GridExporterAddon#175: Related changes to BaseStreamResourceWriter and GridHeader#getText() affecting which text element is selected from multi-row headers.
• FlowingCode/GridExporterAddon#178: Overlapping logic around header text extraction and GridHeader#getText() for joined/multi-row header handling.

Suggested reviewers

• mlopezFC
• javier-godoy

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Correct header rendering for joined columns' clearly and concisely summarizes the main change in the changeset: fixing how headers are rendered when grid columns are joined.
Linked Issues check	✅ Passed	The pull request successfully addresses all objectives from issue #190: CSV/DOCX/PDF exports now use the last header row (closest to data) instead of the first, custom headers apply only to the last header row in all formats, and a demo for joined headers with custom headers was added.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing header rendering for joined columns as specified in issue #190; no out-of-scope modifications were introduced.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-190

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