Add detailed Copilot onboarding instructions for Eclipse Platform UI

.github/copilot-instructions.md

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+# Eclipse Platform UI - Copilot Instructions
+
+## Repository Overview
+
+Eclipse Platform UI provides UI building blocks for Eclipse IDE and Rich Client Platform (RCP): JFace, commands, databinding, dialogs, editors, views, perspectives, and workbench. Built on SWT.
+
+**Key Facts:** 127 MB, 7,675+ Java files, Maven/Tycho build, 57 bundles + 34 test bundles + 25 examples, Java 17, Maven 3.9.x
+
+## Project Structure
+
+### Top-Level Directories
+
+```
+eclipse.platform.ui/
+├── bundles/          # 57 OSGi bundles (production code)
+├── tests/            # 34 test bundles
+├── examples/         # 25 example bundles
+├── features/         # Eclipse feature definitions
+├── releng/           # Release engineering artifacts
+├── docs/             # Extensive documentation (JFace, RCP, Commands, etc.)
+├── .github/          # GitHub workflows and CI configuration
+├── pom.xml           # Root Maven POM
+└── Jenkinsfile       # Jenkins CI configuration
+```
+
+### Key Bundles
+
+- `org.eclipse.ui.workbench` - Main workbench
+- `org.eclipse.jface` - JFace toolkit
+- `org.eclipse.e4.ui.*` - E4 workbench/CSS/DI
+- `org.eclipse.core.databinding*` - Data binding
+- `org.eclipse.core.commands` - Commands
+
+### Bundle Structure
+
+Each OSGi bundle contains: `META-INF/MANIFEST.MF` (dependencies), `build.properties` (build config), `plugin.xml` (extensions), source in `src/` or `eclipseui/`
+
+## Build System - CRITICAL LIMITATIONS
+
+**⚠️ IMPORTANT:** Standalone `mvn clean verify` **FAILS** - requires parent POM from `eclipse.platform.releng.aggregator`
+
+**Workarounds:**
+- Individual bundles: `cd bundles/<bundle>; mvn clean verify -Pbuild-individual-bundles`
+- Full CI build: Uses aggregator project workflows
+- Local testing: Verify syntax/logic without full Maven build
+
+**Maven Config (`.mvn/maven.config`):** `-Pbuild-individual-bundles -Dtycho.target.eager=true -Dtycho.localArtifacts=ignore`
+
+**CI Profiles:** `-Pbree-libs -Papi-check -Pjavadoc` (Jenkins timeout: 80 min)
+
+## Testing
+
+**Test Command:** `mvn clean verify -Pbuild-individual-bundles -DskipTests=false`
+
+**Properties:** `tycho.surefire.useUIHarness=true` (UI test harness), `tycho.surefire.useUIThread=true` (UI thread)
+
+**Dependencies:** Install "Eclipse Test Framework" from release p2 repo for Mockito/Hamcrest
+
+**Test Pattern:**
+```java
+@Before
+public void setUp() {
+    fShell = new Shell(Display.getDefault());
+    // setup
+}
+@After
+public void tearDown() { /* cleanup */ }
+@Test
+public void test() { /* implementation */ }
+```
+
+## CI/GitHub Workflows
+
+**Primary Workflows:**
+- `ci.yml` - Main build (push/PR to master, ignores docs/md), uses aggregator's mavenBuild.yml
+- `pr-checks.yml` - Fast checks (freeze period, merge commits, version increments)
+- `unit-tests.yml` - Publishes test results after CI
+- `codeql.yml` - Security scanning
+
+**Validation Steps:**
+1. Compiler checks (Eclipse compiler)
+2. API compatibility (API tools → `**/target/apianalysis/*.xml`)
+3. Javadoc generation (`failOnJavadocErrors=true`)
+4. Unit tests (JUnit with UI harness)
+5. Test reports (`**/target/surefire-reports/TEST-*.xml`)
+6. Logs archived (`*.log,**/target/**/*.log`)
+
+**CI Environment:** Java 21, xvnc for headless UI tests, quality gate DELTA=1
+
+## Making Code Changes
+
+**Before Starting:** Identify bundle, check `META-INF/MANIFEST.MF` for dependencies, find test bundle
+
+**Common Pitfalls:**
+1. New dependencies → OSGi dependency issues
+2. API changes → Breaks compatibility (CI will fail)
+3. UI code not on Display thread → Crashes
+4. Undisposed SWT resources → Memory leaks
+5. Missing build.properties updates → Build failures
+
+**Critical Patterns:**
+```java
+// Resource disposal
+Display display = Display.getDefault();
+Shell shell = new Shell(display);
+try { /* use */ } finally { shell.dispose(); }
+
+// Async UI
+Display.getDefault().asyncExec(() -> { /* UI code */ });
+
+// Data binding
+DataBindingContext ctx = new DataBindingContext();
+ctx.bindValue(
+    WidgetProperties.text(SWT.Modify).observe(widget),
+    BeanProperties.value("prop").observe(bean)
+);
+```
+
+**Key Files:** `MANIFEST.MF` (dependencies), `plugin.xml` (extensions), `build.properties` (build), `.settings/*.prefs` (compiler)
+
+## Critical Rules
+
+1. Check `MANIFEST.MF` for dependencies before adding imports
+2. Don't break API compatibility - API tools will fail build
+3. Dispose SWT resources (fonts, images, shells) - system colors don't need disposal
+4. UI code must run on Display thread (use `asyncExec`/`syncExec`)
+5. Use existing test infrastructure (JUnit 4, UI harness)
+6. Update `build.properties` when adding files/packages
+7. Follow OSGi patterns - declarative services, no static dependencies
+
+## Quick Reference & Documentation
+
+**Docs:** `docs/JFace.md`, `docs/JFaceDataBinding.md`, `docs/Eclipse4_RCP_FAQ.md`, `docs/PlatformCommandFramework.md`
+**Links:** [Platform UI wiki](https://wiki.eclipse.org/Platform_UI), [Contributing](https://github.com/eclipse-platform/.github/blob/main/CONTRIBUTING.md)
+
+**Root Files:** `.github/` (workflows), `.mvn/` (Maven config), `bundles/` (57), `tests/` (34), `examples/` (25), `features/`, `releng/`, `docs/`, `pom.xml`, `Jenkinsfile`
+
+## Agent Guidance
+
+**Trust these instructions** - validated and tested. Search only if information is incomplete or incorrect.
+
+**Making Changes:**
+1. Identify affected bundle(s)
+2. Review `MANIFEST.MF` dependencies
+3. Find corresponding test bundle
+4. Make minimal, focused changes
+5. Update `build.properties` if adding files
+6. Verify OSGi manifest updates if needed
+
+**Test Failures:**
+- Check UI test needs Display/Shell setup
+- Verify `useUIHarness=true` in test config
+- Check resource disposal
+- Review setup/teardown initialization
+
+**Build Errors:**
+- "Non-resolvable parent POM" → Expected, needs aggregator parent
+- "Package does not exist" → Check `MANIFEST.MF` imports
+- "API baseline errors" → API compatibility broken
+- Test hangs → Missing `Display.syncExec`/`asyncExec`