diff --git a/src/trio/_core/_thread_cache.py b/src/trio/_core/_thread_cache.py
index f48aa8eab..3d044d6de 100644
--- a/src/trio/_core/_thread_cache.py
+++ b/src/trio/_core/_thread_cache.py
@@ -188,6 +188,13 @@ def _handle_job(self) -> None:
             if set_os_thread_name:
                 set_os_thread_name(self._thread.ident, self._default_name)
 
+        # Without this, this thread may get rescheduled after `deliver`
+        # and only return after the kernel gets around to it. That's an
+        # arbitrary amount of time during which we keep anything
+        # referred to in the function alive, like contextvars. That's
+        # surprising!
+        del fn
+
         # Tell the cache that we're available to be assigned a new
         # job. We do this *before* calling 'deliver', so that if
         # 'deliver' triggers a new job, it can be assigned to us
diff --git a/src/trio/_tests/test_threads.py b/src/trio/_tests/test_threads.py
index 532a20d36..f349850ff 100644
--- a/src/trio/_tests/test_threads.py
+++ b/src/trio/_tests/test_threads.py
@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import contextvars
-import gc
 import queue as stdlib_queue
 import re
 import sys
@@ -29,7 +28,7 @@
     sleep_forever,
 )
 from .._core._tests.test_ki import ki_self
-from .._core._tests.tutil import slow
+from .._core._tests.tutil import gc_collect_harder, slow
 from .._threads import (
     active_thread_count,
     current_default_thread_limiter,
@@ -705,7 +704,8 @@ def sync_fn() -> None:
     cvar.set(Foo())
 
     del contextval
-    gc.collect()
+    if sys.implementation.name == "pypy":
+        gc_collect_harder()
 
     assert ref() is None