handle RuntimeError in get_event_loop after asyncio.run

Author: graingert

src/textual/app.py

@@ -2159,7 +2159,17 @@ async def run_app() -> ReturnType | None:
                 # eagerly bind the event loop, and result in Future bound to wrong
                 # loop errors.
                 return asyncio.run(run_app())
-            return asyncio.get_event_loop().run_until_complete(run_app())
+            try:
+                global_loop = asyncio.get_event_loop()
+            except RuntimeError:
+                # the global event loop may have been destroyed by someone running
+                # asyncio.run(), or asyncio.set_event_loop(None), in which case
+                # we need to use asyncio.run() also. (We run this outside the
+                # context of an exception handler)
+                pass
+            else:
+                return global_loop.run_until_complete(run_app())
+            return asyncio.run(run_app())
         return loop.run_until_complete(run_app())
 
     async def _on_css_change(self) -> None:

tests/test_app.py

@@ -367,3 +367,20 @@ def on_mount(self) -> None:
     app = MyApp()
     result = await app.run_async()
     assert result == 42
+
+
+def test_app_loop_run_after_asyncio_run() -> None:
+    """Test that App.run runs after asyncio.run has run."""
+
+    class MyApp(App[int]):
+        def on_mount(self) -> None:
+            self.exit(42)
+
+    async def amain():
+        pass
+
+    asyncio.run(amain())
+
+    app = MyApp()
+    result = app.run()
+    assert result == 42