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UI freezes/deadlocks while debugging apps with many short-lived threads: Debug tree refresh (TreeModelContentProvider.updateNodes) collides with hover #4403
Description
Environment: Eclipse 2026-09 (4.41.0.20260903-0719), Windows Server 2025,
JDK 27; debuggee is Spring Boot 4.1 with virtual threads (enabled by
default), sustained thread churn (scheduled tasks suffice — no external
traffic required).
Symptom: repeated UI freezes (2-30s+, cyclic, CPU only 1-3% during freeze).
Captured deadlock pair in .metadata/.log (2026-09-17/18):
main: TreeModelContentProvider.updateNodes (recursive x3)
-> TreeViewer.replace/setChildCount -> OS.SendMessage (SWT tree surgery)
'Text Viewer Hover Presenter': Display.syncExec() cancelled waiting for the
UI lock
Still reproducible on 4.41 as of 2026-09-24 (monitoring sample attached).
Triggered by thread start/death events while a debug session is active,
even when the user is not interacting.
Workarounds (verified): uncheck Show Running Threads / Thread Groups /
Virtual Threads / Monitors in the Debug view menu, or close the Debug view,
or run without the debugger.
Related:
- Short-lived virtual threads are 1000x slower to create when run in Debug eclipse-jdt/eclipse.jdt.debug#602 (virtual threads 1000x slower under
the debugger) - Virtual threads presentation in Debug View eclipse-jdt/eclipse.jdt.debug#296 (virtual thread presentation in Debug
view — same component) - debug tree is slow with lots of thread creation eclipse-jdt/eclipse.jdt.debug#351 (same component, same stack shape:
TreeModelContentProvider.handleAdd; closed as not planned)
Note: this issue is against the Debug view implementation
(org.eclipse.debug.ui, TreeModelContentProvider); the jdt.debug tickets
above are referenced as the same-family history in the debugger area.
Attached: raw monitoring samples (deadlock pair 2026-09-17, recurrence
2026-09-24).
Activity
fedejeanne commentedon Oct 1, 2026
Hello @lyh061619 , can you please give more details provide samples and/or stack-traces showing the freezes? Is the issue in platform or in Spring? Also, I don't see the relation to #296, maybe your agent meant another repo?
lyh061619 commentedon Oct 2, 2026
Thanks for the triage — answers below; fresh monitoring samples are being
re-captured and will be attached within a day (the freeze reproduces
several times per debugging hour, so capturing is just a matter of letting
a debug session run).
Samples: coming — two fresh captures on their way (the deadlock pair
main-in-TreeModelContentProvider / hover-cancelled-at-syncExec, plus a
standalone recurrence). The 2026-09-18 pair and the 2026-09-24
recurrence from my original report have the same shape; quoting their
key frames here for reference:
main:
TreeModelContentProvider.handleAdd -> updateNodes (x3)
-> TreeViewer.setChildCount -> OS.SendMessage (SWT tree surgery)
'Text Viewer Hover Presenter':
Display.syncExec() cancelled waiting for the UI lock
(SelectedResourceManager.getFromUI <- JavaDebugHover)
Platform or Spring? Platform. The frames are
org.eclipse.debug.ui.viewers (TreeModelContentProvider),
org.eclipse.jface.viewers (TreeViewer) and SWT (OS.SendMessage) — no
Spring/STS frames. The debuggee is Spring Boot 4.1 with virtual
threads on by default, which supplies the thread start/death churn;
any app with many short-lived threads should reproduce. I can attach a
~20-line reproducer (scheduled thread spawner) if useful.
Split packages reported in platform ui #296: yes — Virtual threads presentation in Debug View eclipse-jdt/eclipse.jdt.debug#296, same feature area
(Debug view × virtual threads): that one is about virtual-thread
presentation, this one is thread events flooding the Debug tree
refresh on the UI thread. Related, not duplicate.
Environment: 4.41.0.20260903-0719, Windows Server 2025, JDK 27 (IDE and
debuggee), ~20 Maven projects.
Workaround in daily use (verified): uncheck Show Running Threads / Thread
Groups / Virtual Threads / Monitors in the Debug view menu, or close the
Debug view while the app runs.