TestAssemblySetup set the unhandled-exception mode and a ThreadException
handler from a module initializer. WinForms stores both per thread
(ThreadContext/NativeWindow thread statics), so they applied only to the
runner thread that loaded the assembly, never to the STA threads the
tests run on. There, an exception escaping a window message or a
BeginInvoke/SynchronizationContext callback still opened a modal
ThreadExceptionDialog in an interactive session, or was silently ignored
otherwise; the handler would also have logged and swallowed it.
Replace the fifteen copied RunSta helpers with StaTestThread.Run, which
selects UnhandledExceptionMode.ThrowException on each STA thread before
it creates a window, so the exception propagates to the test body and
fails the test. Each class keeps its own timeout and message; all
threads are now background threads. Remove TestAssemblySetup.
StaTestThreadTests covers a throwing BeginInvoke callback through
StaTestThread (fails with the original exception) and, as a control,
the default mode on a plain STA thread (the exception goes to the
thread's handler and never reaches the test body). Cross-compiled on
Linux; Windows execution is the windows-desktop CI job.
The reorder event snapshot could previously show the reverse pass's
stale timeout label. Set a sentinel before each click so the
ProgramChanged handler can only observe the label applied by the
current operation's highlighting pass.
Quality review of 50884da found the unshown RichTextBox oracle compared
cached CRLF text against the highlighted editor's native LF-normalized
text, which would fail on first Windows execution. Compare generated
text with normalized line endings on both sides instead, capture the
status label and preview state inside the ProgramChanged handler to
prove fallback-before-notify ordering, document the HighlightSpan
UTF-16 and rule-index contract, and pin the null-text argument check.
aj