Add container 'Middle'.

Related issues: #1957.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added `HorizontalScroll` https://github.com/Textualize/textual/issues/1957
 - Added `Center` https://github.com/Textualize/textual/issues/1957
+- Added `Middle` https://github.com/Textualize/textual/issues/1957
 
 
 ## [0.14.0] - 2023-03-09

src/textual/containers.py

@@ -61,6 +61,18 @@ class Center(Widget):
     """
 
 
+class Middle(Widget):
+    """A container widget which aligns children vertically in the middle."""
+
+    DEFAULT_CSS = """
+    Middle {
+        align-vertical: middle;
+        height: 100%;
+        width: auto;
+    }
+    """
+
+
 class Grid(Widget):
     """A container widget with grid alignment."""
 

tests/test_containers.py

@@ -1,7 +1,7 @@
 """Test basic functioning of some containers."""
 
 from textual.app import App, ComposeResult
-from textual.containers import Center, Horizontal, HorizontalScroll
+from textual.containers import Center, Horizontal, HorizontalScroll, Middle
 from textual.widgets import Label
 
 
@@ -47,3 +47,18 @@ async def test_center_container():
         center = app.query_one(Center)
         assert center.size.width == app.size.width
         assert center.size.height == 3
+
+
+async def test_middle_container():
+    """Check the size of the container `Middle`."""
+
+    class MiddleApp(App[None]):
+        def compose(self) -> ComposeResult:
+            with Middle():
+                yield Label("1234")
+
+    app = MiddleApp()
+    async with app.run_test():
+        middle = app.query_one(Middle)
+        assert middle.size.width == 4
+        assert middle.size.height == app.size.height