org.eclipse.lsp4e.outline.HasCNFOutlinePage.test can freeze the UI when an editor is opened

We have observed the following UI freeze caused by LSP4E:

!ENTRY org.eclipse.ui.monitoring 2 0 2025-07-21 10:02:20.925
!MESSAGE UI freeze of 0.31s at 10:02:20.596
!SUBENTRY 1 org.eclipse.ui.monitoring 1 0 2025-07-21 10:02:20.925
!MESSAGE Sample at 10:02:20.797 (+0.201s)
Thread 'main' tid=1 (TIMED_WAITING)
!STACK 0
Stack Trace
	at java.base@17.0.5/jdk.internal.misc.Unsafe.park(Native Method)
	at java.base@17.0.5/java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:252)
	at java.base@17.0.5/java.util.concurrent.CompletableFuture$Signaller.block(CompletableFuture.java:1866)
	at java.base@17.0.5/java.util.concurrent.ForkJoinPool.unmanagedBlock(ForkJoinPool.java:3463)
	at java.base@17.0.5/java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3434)
	at java.base@17.0.5/java.util.concurrent.CompletableFuture.timedGet(CompletableFuture.java:1939)
	at java.base@17.0.5/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2095)
	at org.eclipse.lsp4e.outline.EditorToOutlineAdapterFactory.getAdapter(EditorToOutlineAdapterFactory.java:68)
	at org.eclipse.core.internal.adapter.AdapterFactoryProxy.lambda$5(AdapterFactoryProxy.java:111)
	at org.eclipse.core.internal.adapter.AdapterFactoryProxy$$Lambda$697/0x0000000800d336a0.apply(Unknown Source)
	at java.base@17.0.5/java.util.Optional.map(Optional.java:260)
	at org.eclipse.core.internal.adapter.AdapterFactoryProxy.getAdapter(AdapterFactoryProxy.java:111)
	at org.eclipse.core.internal.runtime.AdapterManager.lambda$7(AdapterManager.java:253)
	at org.eclipse.core.internal.runtime.AdapterManager$$Lambda$680/0x00000008012fced0.apply(Unknown Source)
	at java.base@17.0.5/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
	at java.base@17.0.5/java.util.ArrayList$ArrayListSpliterator.tryAdvance(ArrayList.java:1602)
	at java.base@17.0.5/java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:129)
	at java.base@17.0.5/java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:527)
	at java.base@17.0.5/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:513)
	at java.base@17.0.5/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
	at java.base@17.0.5/java.util.stream.FindOps$FindOp.evaluateSequential(FindOps.java:150)
	at java.base@17.0.5/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base@17.0.5/java.util.stream.ReferencePipeline.findFirst(ReferencePipeline.java:647)
	at org.eclipse.core.internal.runtime.AdapterManager.getAdapter(AdapterManager.java:266)
	at org.eclipse.ui.part.WorkbenchPart.getAdapter(WorkbenchPart.java:152)
	at org.eclipse.ui.texteditor.AbstractTextEditor.getAdapter(AbstractTextEditor.java:6196)
	at org.eclipse.ui.texteditor.AbstractDecoratedTextEditor.getAdapter(AbstractDecoratedTextEditor.java:1479)
	at org.eclipse.ui.editors.text.TextEditor.getAdapter(TextEditor.java:163)
	at org.eclipse.lsp4e.outline.HasCNFOutlinePage.test(HasCNFOutlinePage.java:28)
	at org.eclipse.core.internal.expressions.Property.test(Property.java:65)
	at org.eclipse.core.expressions.TestExpression.evaluate(TestExpression.java:107)
	at org.eclipse.core.expressions.CompositeExpression.evaluateAnd(CompositeExpression.java:54)
	at org.eclipse.core.expressions.WithExpression.evaluate(WithExpression.java:84)
	at org.eclipse.core.expressions.ReferenceExpression.evaluate(ReferenceExpression.java:72)
	at org.eclipse.e4.ui.internal.workbench.ContributionsAnalyzer.isCoreExpressionVisible(ContributionsAnalyzer.java:263)
	at org.eclipse.e4.ui.internal.workbench.ContributionsAnalyzer.isVisible(ContributionsAnalyzer.java:237)
	at org.eclipse.e4.ui.workbench.renderers.swt.ContributionRecord.computeVisibility(ContributionRecord.java:167)
	at org.eclipse.e4.ui.workbench.renderers.swt.ContributionRecord.updateVisibility(ContributionRecord.java:98)
	at org.eclipse.e4.ui.workbench.renderers.swt.MenuManagerRenderer$1.changed(MenuManagerRenderer.java:539)
	at org.eclipse.e4.core.internal.contexts.TrackableComputationExt.update(TrackableComputationExt.java:105)
	at org.eclipse.e4.core.internal.contexts.EclipseContext.processScheduled(EclipseContext.java:358)
	at org.eclipse.e4.core.internal.contexts.EclipseContext.set(EclipseContext.java:374)
	at org.eclipse.ui.internal.WorkbenchPage.updateActiveEditorSources(WorkbenchPage.java:414)
	at org.eclipse.ui.internal.WorkbenchPage.updateBroughtToTop(WorkbenchPage.java:464)
	at org.eclipse.ui.internal.WorkbenchPage$E4PartListener.partBroughtToTop(WorkbenchPage.java:221)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl$6.run(PartServiceImpl.java:314)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl.firePartBroughtToTop(PartServiceImpl.java:311)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl.lambda$0(PartServiceImpl.java:107)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl$$Lambda$490/0x00000008012cca58.handleEvent(Unknown Source)
	at org.eclipse.e4.ui.services.internal.events.UIEventHandler.lambda$0(UIEventHandler.java:38)
	at org.eclipse.e4.ui.services.internal.events.UIEventHandler$$Lambda$430/0x00000008011b1aa8.run(Unknown Source)
	at org.eclipse.swt.widgets.Synchronizer.syncExec(Synchronizer.java:183)
	at org.eclipse.ui.internal.UISynchronizer.syncExec(UISynchronizer.java:133)
	at org.eclipse.swt.widgets.Display.syncExec(Display.java:4941)
	at org.eclipse.e4.ui.workbench.swt.DisplayUISynchronize.syncExec(DisplayUISynchronize.java:34)
	at org.eclipse.e4.ui.services.internal.events.UIEventHandler.handleEvent(UIEventHandler.java:38)
	at org.eclipse.equinox.internal.event.EventHandlerWrapper.handleEvent(EventHandlerWrapper.java:206)
	at org.eclipse.equinox.internal.event.EventHandlerTracker.dispatchEvent(EventHandlerTracker.java:201)
	at org.eclipse.equinox.internal.event.EventHandlerTracker.dispatchEvent(EventHandlerTracker.java:1)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:230)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151)
	at org.eclipse.equinox.internal.event.EventAdminImpl.dispatchEvent(EventAdminImpl.java:131)
	at org.eclipse.equinox.internal.event.EventAdminImpl.sendEvent(EventAdminImpl.java:73)
	at org.eclipse.equinox.internal.event.EventComponent.sendEvent(EventComponent.java:44)
	at org.eclipse.e4.ui.services.internal.events.EventBroker.send(EventBroker.java:55)
	at org.eclipse.e4.ui.internal.workbench.UIEventPublisher.notifyChanged(UIEventPublisher.java:60)
	at org.eclipse.emf.common.notify.impl.BasicNotifierImpl.eNotify(BasicNotifierImpl.java:424)
	at org.eclipse.e4.ui.model.application.ui.impl.ElementContainerImpl.setSelectedElementGen(ElementContainerImpl.java:168)
	at org.eclipse.e4.ui.model.application.ui.impl.ElementContainerImpl.setSelectedElement(ElementContainerImpl.java:187)
	at org.eclipse.e4.ui.internal.workbench.ModelServiceImpl.showElementInWindow(ModelServiceImpl.java:654)
	at org.eclipse.e4.ui.internal.workbench.ModelServiceImpl.bringToTop(ModelServiceImpl.java:618)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl.delegateBringToTop(PartServiceImpl.java:796)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl.bringToTop(PartServiceImpl.java:401)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl.showPart(PartServiceImpl.java:1271)
	at org.eclipse.ui.internal.WorkbenchPage.busyOpenEditor(WorkbenchPage.java:3228)
	at org.eclipse.ui.internal.WorkbenchPage.lambda$11(WorkbenchPage.java:3118)
	at org.eclipse.ui.internal.WorkbenchPage$$Lambda$1156/0x00000008017cab98.run(Unknown Source)
	at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:67)
	at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:3116)
...

Most probably this can be avoided if the tester does not ask for the outline page as in

IContentOutlinePage outlinePage = editor.getAdapter(IContentOutlinePage.class);
return outlinePage instanceof CNFOutlinePage;

and instead it has checks if there is a language server with the capability ServerCapabilities::getDocumentSymbolProvider and it is active.

Start with org.eclipse.lsp4e.outline.HasCNFOutlinePage.test and inspect the adapter call at org.eclipse.lsp4e.outline.EditorToOutlineAdapterFactory.java:68. Trace how the synchronous outline lookup interacts with language-server capability and activity checks. Done means opening an editor no longer causes this UI freeze while the outline-page test still behaves correctly.