Add regression test for #3931.

tests/css/css_reloading.tcss

@@ -0,0 +1 @@
+/* This file has no rules intentionally. */

tests/css/test_css_reloading.py

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+"""
+Regression test for https://github.com/Textualize/textual/issues/3931
+"""
+
+from pathlib import Path
+
+from textual.app import App, ComposeResult
+from textual.screen import Screen
+from textual.widgets import Label
+
+CSS_PATH = (Path(__file__) / "../css_reloading.tcss").resolve()
+
+Path(CSS_PATH).write_text(
+    """\
+Label {
+    height: 5;
+    border: panel white;
+}
+"""
+)
+
+
+class BaseScreen(Screen[None]):
+    def compose(self) -> ComposeResult:
+        yield Label("I am the base screen")
+
+
+class TopScreen(Screen[None]):
+    DEFAULT_CSS = """
+    TopScreen {
+        opacity: 1;
+        background: green 0%;
+    }
+    """
+
+
+class MyApp(App[None]):
+    CSS_PATH = CSS_PATH
+
+    def on_mount(self) -> None:
+        self.push_screen(BaseScreen())
+        self.push_screen(TopScreen())
+
+
+async def test_css_reloading_applies_to_non_top_screen(monkeypatch) -> None:  # type: ignore
+    """Regression test for https://github.com/Textualize/textual/issues/2063."""
+
+    monkeypatch.setenv(
+        "TEXTUAL", "debug"
+    )  # This will make sure we create a file monitor.
+
+    app = MyApp()
+    async with app.run_test() as pilot:
+        await pilot.pause()
+        first_label = pilot.app.screen_stack[-2].query(Label).first()
+        # Sanity check.
+        assert first_label.styles.height is not None
+        assert first_label.styles.height.value == 5
+
+        # Clear the CSS from the file.
+        Path(CSS_PATH).write_text("/* This file has no rules intentionally. */\n")
+        await pilot.app._on_css_change()
+        # Height should fall back to 1.
+        assert first_label.styles.height is not None
+        assert first_label.styles.height.value == 1