Fix alignment in auto container

src/textual/layout.py

@@ -259,18 +259,7 @@ def get_content_height(
         if not widget._nodes:
             height = 0
         else:
-            # Use a height of zero to ignore relative heights
-            styles_height = widget.styles.height
-            if widget._parent and len(widget._nodes) == 1:
-                # If it is an only child with height auto we want it to expand
-                height = (
-                    container.height
-                    if styles_height is not None and styles_height.is_auto
-                    else 0
-                )
-            else:
-                height = 0
-            arrangement = widget._arrange(Size(width, height))
+            arrangement = widget._arrange(Size(width, 0))
             height = arrangement.total_region.bottom
 
         return height

tests/snapshot_tests/snapshot_apps/max_height_100.py

@@ -3,9 +3,12 @@
 
 
 class HappyDataTableFunApp(App[None]):
-    """The DataTable should expand as if it has height 1fr."""
+    """The DataTable should expand to full the screen and show a horizontal scrollbar."""
 
     CSS = """
+    #s {
+        max-height: 100%;
+    }
     DataTable {
         max-height: 100%;        
     }

tests/snapshot_tests/test_snapshots.py

@@ -1385,7 +1385,7 @@ def test_vertical_max_height(snap_compare):
 
 
 def test_max_height_100(snap_compare):
-    """Test vertical max height takes border in to account."""
+    """Test a datatable with max height 100%."""
     assert snap_compare(SNAPSHOT_APPS_DIR / "max_height_100.py")
 
 
@@ -3074,3 +3074,40 @@ def compose(self):
             yield MainContainer()
 
     snap_compare(Test1())
+
+
+def test_auto_parent_with_alignment(snap_compare):
+    class Sidebar(Vertical):
+        DEFAULT_CSS = """
+        Sidebar {
+            dock: right;
+            width: auto;
+            height: auto;
+            background: blue;
+            align-vertical: bottom;
+
+            #contents {
+                width: auto;
+                height: auto;
+                background: red;
+                border: white;
+            }        
+        }
+        """
+
+        def compose(self) -> ComposeResult:
+            with Vertical(id="contents"):
+                yield Button("Start")
+                yield Button("Stop")
+
+    class FloatSidebarApp(App):
+        CSS = """
+        Screen {
+            layers: base sidebar;
+        }
+        """
+
+        def compose(self) -> ComposeResult:
+            yield Sidebar()
+
+    snap_compare(FloatSidebarApp())