prevent error in TextArea constructor

CHANGELOG.md

@@ -7,9 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
-### Fixed
+### [0.80.1] - 2024-09-24
 
 - Fixed crash when exiting the app prematurely https://github.com/Textualize/textual/pull/5039
+- Fixed exception constructing TextArea outside of App https://github.com/Textualize/textual/pull/5045
 
 ## [0.80.0] - 2024-09-23
 
@@ -2387,6 +2388,7 @@ https://textual.textualize.io/blog/2022/11/08/version-040/#version-040
 - New handler system for messages that doesn't require inheritance
 - Improved traceback handling
 
+[0.80.1]: https://github.com/Textualize/textual/compare/v0.80.0...v0.80.1
 [0.80.0]: https://github.com/Textualize/textual/compare/v0.79.0...v0.80.0
 [0.79.0]: https://github.com/Textualize/textual/compare/v0.78.0...v0.79.0
 [0.78.0]: https://github.com/Textualize/textual/compare/v0.77.0...v0.78.0

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual"
-version = "0.80.0"
+version = "0.80.1"
 homepage = "https://github.com/Textualize/textual"
 repository = "https://github.com/Textualize/textual"
 documentation = "https://textual.textualize.io/"

src/textual/widgets/_text_area.py

@@ -464,9 +464,6 @@ def __init__(
 
         self.tab_behavior = tab_behavior
 
-        # When `app.dark` is toggled, reset the theme (since it caches values).
-        self.watch(self.app, "dark", self._app_dark_toggled, init=False)
-
         if tooltip is not None:
             self.tooltip = tooltip
 
@@ -610,6 +607,10 @@ def _watch_selection(
     ) -> None:
         """When the cursor moves, scroll it into view."""
         # Find the visual offset of the cursor in the document
+
+        if not self.is_mounted:
+            return
+
         cursor_location = selection.end
 
         self.scroll_cursor_visible()
@@ -1519,6 +1520,9 @@ def gutter_width(self) -> int:
         return gutter_width
 
     def _on_mount(self, event: events.Mount) -> None:
+        # When `app.dark` is toggled, reset the theme (since it caches values).
+        self.watch(self.app, "dark", self._app_dark_toggled, init=False)
+
         self.blink_timer = self.set_interval(
             0.5,
             self._toggle_cursor_blink_visible,

tests/test_widget.py

@@ -12,7 +12,46 @@
 from textual.geometry import Offset, Size
 from textual.message import Message
 from textual.widget import BadWidgetName, MountError, PseudoClasses, Widget
-from textual.widgets import Label, LoadingIndicator
+from textual.widgets import (
+    Button,
+    DataTable,
+    Footer,
+    Header,
+    Input,
+    Label,
+    LoadingIndicator,
+    Log,
+    OptionList,
+    RichLog,
+    Switch,
+    TextArea,
+)
+
+
+async def test_widget_construct():
+    """Regression test for https://github.com/Textualize/textual/issues/5042"""
+
+    # Check that constructing the widget outside of the app, doesn't invoke code that
+    # expects an active app.
+    class MyApp(App):
+        def __init__(self) -> None:
+            super().__init__()
+            self.button = Button()
+            self.data_table = DataTable()
+            self.footer = Footer()
+            self.header = Header()
+            self.input = Input()
+            self.label = Label()
+            self.loading_indicator = LoadingIndicator()
+            self.log = Log()
+            self.option_list = OptionList()
+            self.rich_log = RichLog()
+            self.switch = Switch()
+            self.text_area = TextArea(language="python")
+
+    app = MyApp()
+    async with app.run_test():
+        pass
 
 
 @pytest.mark.parametrize(