Fix #6444: Crash when using Input in a DEFAULT_MODE screen

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+
+- Fix crash when using Input in a DEFAULT_MODE screen https://github.com/Textualize/textual/pull/6492
+
 ### Added
 
 - Added `DOM.update_classes` https://github.com/Textualize/textual/pull/6478

src/textual/widgets/_input.py

@@ -252,7 +252,7 @@ def cursor_position(self, position: int) -> None:
         """Set the current position of the cursor."""
         self.selection = Selection.cursor(position)
 
-    selection: Reactive[Selection] = reactive(Selection.cursor(0))
+    selection: Reactive[Selection] = reactive(Selection.cursor(0), init=False)
     """The currently selected range of text."""
 
     placeholder = reactive("")
@@ -505,6 +505,9 @@ def validate_selection(self, selection: Selection) -> Selection:
         return Selection(clamp(start, 0, value_length), clamp(end, 0, value_length))
 
     def _watch_selection(self, selection: Selection) -> None:
+        if not self.is_mounted:
+            return
+
         self.app.clear_selection()
         self.app.cursor_position = self.cursor_screen_offset
         if not self._initial_value:

tests/input/test_input_default_mode.py

@@ -0,0 +1,34 @@
+import asyncio
+
+import pytest
+
+from textual.app import App, ComposeResult
+from textual.screen import Screen
+from textual.widgets import Input
+
+
+class DefaultScreen(Screen):
+    def compose(self) -> ComposeResult:
+        yield Input()
+        yield Input("Hello, world!")
+
+
+class InputApp(App):
+    DEFAULT_MODE = "default"
+    MODES = {"default": DefaultScreen}
+
+
+@pytest.mark.skipif(
+    not hasattr(asyncio, "eager_task_factory"), reason="only occurs with eager tasks"
+)
+async def test_input_default_mode():
+    """Test that Input in a DEFAULT_MODE Screen doesn't crash.
+
+    Regression test for https://github.com/textualize/textual/issues/6444.
+
+    """
+    # changing this will not affect other tests since pytest-asyncio runs every function
+    # in a new event loop by default
+    asyncio.get_running_loop().set_task_factory(asyncio.eager_task_factory)
+    async with InputApp().run_test():
+        pass