The crash is a NullReferenceException in GetNodeByVisibleIndex, reached when a
background decompile realizes a node's children while the UI thread is indexing
the flattener. Eight ILSpyTreeNode.Decompile overrides call EnsureLazyChildren
from that task; two wrapped it in Dispatcher.UIThread.Invoke, six did not, and
one of the two lost its wrapper in the Avalonia port with no test noticing for a
release cycle. A rule every call site has to remember is a rule that gets broken
again, so EnsureLazyChildren marshals itself instead: SetOwner already named the
owning thread, and now also carries the host's way onto it. A call already on the
owner runs inline, so a blocking invoke cannot deadlock on itself and a nested
load costs no further hop; an unowned tree is left unmarshalled, which keeps
building a subtree on a worker and publishing it on the UI thread legal.
The affinity check stays as the regression detector, but its fail-fast throw was
worthless on its own: tree mutation happens inside callers that catch Exception,
so the throw ended up rendered into the decompiled output and the run passed. The
violation is now recorded before the throw, and an assembly-level NUnit test
action fails the test that produced one - an assembly-level teardown failure is
reported but leaves the exit code at zero.
Assisted-by: Claude:claude-opus-5:Claude Code
Siegfried Pammer
228041085b