[scroll_to_widget] Add a quick sandbox to test the feature

sandbox/scroll_to_widget.py

@@ -0,0 +1,72 @@
+from rich.console import RenderableType
+from rich.text import Text
+
+from textual.app import App, ComposeResult
+from textual.widget import Widget
+from textual.widgets import Placeholder
+
+placeholders_count = 12
+
+
+class VerticalContainer(Widget):
+    CSS = """
+    VerticalContainer {
+        layout: vertical;
+        overflow: hidden auto;
+        background: darkblue;
+    }
+
+    VerticalContainer Placeholder {
+        margin: 1 0;
+        height: 5;
+        border: solid lime;
+        align: center top;
+    }
+    """
+
+
+class Introduction(Widget):
+    CSS = """
+    Introduction {
+        background: indigo;
+        color: white;
+        height: 3;
+        padding: 1 0;
+    }
+    """
+
+    def render(self) -> RenderableType:
+        return Text(
+            "Press keys 0 to 9 to scroll to the Placeholder with that ID.",
+            justify="center",
+        )
+
+
+class MyTestApp(App):
+    def compose(self) -> ComposeResult:
+        placeholders = [
+            Placeholder(id=f"placeholder_{i}", name=f"Placeholder #{i}")
+            for i in range(placeholders_count)
+        ]
+
+        yield VerticalContainer(Introduction(), *placeholders, id="root")
+
+    def on_mount(self):
+        self.bind("q", "quit")
+        self.bind("t", "tree")
+        for widget_index in range(placeholders_count):
+            self.bind(str(widget_index), f"scroll_to('placeholder_{widget_index}')")
+
+    def action_tree(self):
+        self.log(self.tree)
+
+    async def action_scroll_to(self, target_placeholder_id: str):
+        target_placeholder = self.query(f"#{target_placeholder_id}").first()
+        target_placeholder_container = self.query("#root").first()
+        target_placeholder_container.scroll_to_widget(target_placeholder, animate=True)
+
+
+app = MyTestApp()
+
+if __name__ == "__main__":
+    app.run()