Yield to give "synchronous" writes a chance to complete

shiny/reactive/_reactives.py

@@ -15,6 +15,7 @@
     "event",
 )
 
+import asyncio
 import functools
 import traceback
 import warnings
@@ -575,6 +576,10 @@ async def _run(self) -> None:
             try:
                 with ctx():
                     await self._fn()
+
+                    # Yield so that messages can be sent to the client if necessary.
+                    # https://github.com/posit-dev/py-shiny/issues/1381
+                    await asyncio.sleep(0)
             except SilentException:
                 # It's OK for SilentException to cause an Effect to stop running
                 pass

shiny/session/_session.py

@@ -4,6 +4,7 @@
 
 __all__ = ("Session", "Inputs", "Outputs")
 
+import asyncio
 import contextlib
 import dataclasses
 import enum
@@ -653,6 +654,12 @@ def verify_state(expected_state: ConnectionState) -> None:
                                 f"Unrecognized method {message_obj['method']}"
                             )
 
+                        # Progress messages (of the "{binding: {id: xxx}}"" variety) may
+                        # have queued up at this point; let them drain before we send
+                        # the next message.
+                        # https://github.com/posit-dev/py-shiny/issues/1381
+                        await asyncio.sleep(0)
+
                         self._request_flush()
 
                         await flush()

tests/pytest/mocktime.py

@@ -67,6 +67,16 @@ async def _sleep(self, delay: float) -> None:
         self._i += 1
         # Oldest first
         self._sleepers.sort()
+
+        # This is necessary for some tests that call logic that internally awaits on
+        # asyncio.sleep(0). Without this, they hang.
+        #
+        # Another potential workaround would've been to check if delay <= 0 and just
+        # return. But this solution has the nice property of actually yielding control
+        # (I think...!), which is the whole point of asyncio.sleep(0).
+        if not self._is_advancing:
+            await self.advance_time(0)
+
         await wake.wait()