From 289ac618a56b58807ff32db53aea7a3ef0c2b211 Mon Sep 17 00:00:00 2001
From: Paul Ollis <paul@cleversheep.org>
Date: Sun, 20 Apr 2025 17:38:42 +0100
Subject: [PATCH] Create new event loop as fallback behaviour.

The method App.push_screen tries to create a Future using
`asyncio.get_running_loop()`. If this raises a RuntimeError it simply
invokes `Future()`, as a fallback, order to support some of the tests.

When running tests using pytest-xdist it is possible for this fallback
mechanism to fail and instead raise another RuntimeError when `Future()`
tries to get the 'standard' event loop. This is a rare occurrence with
the current tests' structure, but I have seen it a number of times.

It is causes by a test that invokes App.run (which invokes asyncio.run)
being executed before a test that depends on the aforementioned fallback
mechanism. Both tests must be executed by the same pytest-xdist worker
process.
---
 src/textual/app.py | 33 ++++++++++++++++++---------------
 1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/src/textual/app.py b/src/textual/app.py
index 6861012863..e5c4e4f087 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -314,8 +314,8 @@ class App(Generic[ReturnType], DOMNode):
                 scrollbar-background-active: ansi_default;
                 scrollbar-color: ansi_blue;
                 scrollbar-color-active: ansi_bright_blue;
-                scrollbar-color-hover: ansi_bright_blue;    
-                scrollbar-corner-color: ansi_default;           
+                scrollbar-color-hover: ansi_bright_blue;
+                scrollbar-corner-color: ansi_default;
             }
 
             .bindings-table--key {
@@ -336,18 +336,18 @@ class App(Generic[ReturnType], DOMNode):
         }
 
         /* When a widget is maximized */
-        Screen.-maximized-view {                    
+        Screen.-maximized-view {
             layout: vertical !important;
             hatch: right $panel;
             overflow-y: auto !important;
             align: center middle;
             .-maximized {
-                dock: initial !important;                
+                dock: initial !important;
             }
         }
         /* Fade the header title when app is blurred */
-        &:blur HeaderTitle {           
-            text-opacity: 50%;           
+        &:blur HeaderTitle {
+            text-opacity: 50%;
         }
     }
     *:disabled:can-focus {
@@ -399,7 +399,7 @@ class MyApp(App[None]):
 
     ALLOW_SELECT: ClassVar[bool] = True
     """A switch to toggle arbitrary text selection for the app.
-    
+
     Note that this doesn't apply to Input and TextArea which have builtin support for selection.
     """
 
@@ -445,7 +445,7 @@ class MyApp(App[None]):
     """The default value of [Screen.ALLOW_IN_MAXIMIZED_VIEW][textual.screen.Screen.ALLOW_IN_MAXIMIZED_VIEW]."""
 
     CLICK_CHAIN_TIME_THRESHOLD: ClassVar[float] = 0.5
-    """The maximum number of seconds between clicks to upgrade a single click to a double click, 
+    """The maximum number of seconds between clicks to upgrade a single click to a double click,
     a double click to a triple click, etc."""
 
     BINDINGS: ClassVar[list[BindingType]] = [
@@ -472,7 +472,7 @@ class MyApp(App[None]):
 
     ESCAPE_TO_MINIMIZE: ClassVar[bool] = True
     """Use escape key to minimize widgets (potentially overriding bindings).
-    
+
     This is the default value, used if the active screen's `ESCAPE_TO_MINIMIZE` is not changed from `None`.
     """
 
@@ -544,7 +544,7 @@ def __init__(
 
         self._registered_themes: dict[str, Theme] = {}
         """Themes that have been registered with the App using `App.register_theme`.
-        
+
         This excludes the built-in themes."""
 
         for theme in BUILTIN_THEMES.values():
@@ -746,7 +746,7 @@ def __init__(
 
         self.theme_changed_signal: Signal[Theme] = Signal(self, "theme-changed")
         """Signal that is published when the App's theme is changed.
-        
+
         Subscribers will receive the new theme object as an argument to the callback.
         """
 
@@ -2681,10 +2681,13 @@ def push_screen(
         try:
             loop = asyncio.get_running_loop()
         except RuntimeError:
-            # Mainly for testing, when push_screen isn't called in an async context
-            future: asyncio.Future[ScreenResultType] = asyncio.Future()
-        else:
-            future = loop.create_future()
+            # Mainly for testing, when push_screen isn't called in an async context.
+            # Note:
+            #     Creating a new event loop is necessary. Simply calling Future()
+            #     without a loop parameter can fail when running tests using
+            #     pytest-xdist.
+            loop = asyncio.new_event_loop()
+        future = loop.create_future()
 
         if self._screen_stack:
             self.screen.post_message(events.ScreenSuspend())