Layer scroll fix (#2196)

* fix scroll spacing

* fix and snapshot

* changelog

tests/snapshot_tests/snapshot_apps/dock_scroll.py

@@ -0,0 +1,31 @@
+from textual.app import App
+from textual.widgets import Header, Label, Footer
+
+
+class TestApp(App):
+    BINDINGS = [("ctrl+q", "app.quit", "Quit")]
+    CSS = """
+    
+    Label {
+        border: solid red;
+    }
+    Footer {
+        height: 4;
+    }
+    """
+
+    def compose(self):
+        text = (
+            "this is a sample sentence and here are some words".replace(" ", "\n") * 2
+        )
+        yield Header()
+        yield Label(text)
+        yield Footer()
+
+    def on_mount(self):
+        self.dark = False
+
+
+if __name__ == "__main__":
+    app = TestApp()
+    app.run()