[layouts] Address feedback on "Fix vertical layout bug with centered content" PR

tests/test_integration_layout.py

@@ -1,6 +1,5 @@
 from __future__ import annotations
 import asyncio
-import platform
 from typing import cast, List
 
 import pytest
@@ -11,9 +10,6 @@ from textual.geometry import Size
 from textual.widget import Widget
 from textual.widgets import Placeholder
 
-PLATFORM = platform.system()
-WINDOWS = PLATFORM == "Windows"
-
 # Let's allow ourselves some abbreviated names for those tests,
 # in order to make the test cases a bit easier to read :-)
 SCREEN_W = 100  # width of our Screens
@@ -153,22 +149,11 @@ async def test_composition_of_vertical_container_with_children(
 
     expected_screen_size = Size(*screen_size)
 
-    # On Windows the screen size is always 1 cell smaller than the requested dimensions.
-    # Let's take this into account into our "expected_*" measurements:
-    if WINDOWS:
-        expected_screen_size = expected_screen_size._replace(
-            width=expected_screen_size.width - 1
-        )
-        expected_placeholders_size = (
-            expected_placeholders_size[0] - 1,
-            expected_placeholders_size[1],
-        )
-
     async with app.in_running_state():
         app.log_tree()
 
         # root widget checks:
-        root_widget = cast(Widget, app.query_one("#root"))
+        root_widget = cast(Widget, app.get_child("root"))
         assert root_widget.size == expected_screen_size
         root_widget_region = app.screen.get_widget_region(root_widget)
         assert root_widget_region == (