Add 'HorizontalScroll'.

Related issues: #1957.

tests/test_containers.py

@@ -0,0 +1,34 @@
+"""Test basic functioning of some containers."""
+
+from textual.app import App, ComposeResult
+from textual.containers import Horizontal, HorizontalScroll
+from textual.widgets import Label
+
+
+async def test_horizontal_vs_horizontalscroll_scrolling():
+    """Check the default scrollbar behaviours for Horizontal and HorizontalScroll."""
+
+    class HorizontalsApp(App[None]):
+        CSS = """
+        Screen {
+            layout: vertical;
+        }
+        """
+
+        def compose(self) -> ComposeResult:
+            with Horizontal():
+                for _ in range(10):
+                    yield Label("How is life going? " * 3 + " | ")
+            with HorizontalScroll():
+                for _ in range(10):
+                    yield Label("How is life going? " * 3 + " | ")
+
+    WIDTH = 80
+    HEIGHT = 24
+    app = HorizontalsApp()
+    async with app.run_test(size=(WIDTH, HEIGHT)):
+        horizontal = app.query_one(Horizontal)
+        horizontal_scroll = app.query_one(HorizontalScroll)
+        assert horizontal.size.height == horizontal_scroll.size.height
+        assert horizontal.scrollbars_enabled == (False, False)
+        assert horizontal_scroll.scrollbars_enabled == (False, True)