Add test for scroll_to_center method.

tests/snapshot_tests/snapshot_apps/scroll_to_center.py

@@ -0,0 +1,40 @@
+from textual.app import App, ComposeResult
+from textual.containers import HorizontalScroll, VerticalScroll
+from textual.widgets import Label
+
+
+class MyApp(App[None]):
+    CSS = """
+    VerticalScroll, HorizontalScroll {
+        border: round $primary;
+    }
+    #vertical {
+        height: 21;
+    }
+    HorizontalScroll {
+        height: auto;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        with VerticalScroll():
+            yield Label(("SPAM\n" * 25)[:-1])
+            with VerticalScroll():
+                yield Label(("SPAM\n" * 53)[:-1])
+                with VerticalScroll(id="vertical"):
+                    yield Label(("SPAM\n" * 78)[:-1])
+                    with HorizontalScroll():
+                        yield Label(("v\n" * 17)[:-1])
+                        yield Label("@" * 302)
+                        yield Label("[red]>>bullseye<<[/red]", id="bullseye")
+                        yield Label("@" * 99)
+                    yield Label(("SPAM\n" * 49)[:-1])
+                yield Label(("SPAM\n" * 51)[:-1])
+            yield Label(("SPAM\n" * 59)[:-1])
+
+    def key_s(self) -> None:
+        self.query_one("#bullseye").scroll_to_center()
+
+
+if __name__ == "__main__":
+    MyApp().run()