Add sandbox code for testing work on #939

sandbox/davep/Makefile

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+all:
+	poetry run textual run --dev focus_removal_tester.py

sandbox/davep/focus_removal_tester.py

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+"""Focus removal tester.
+
+https://github.com/Textualize/textual/issues/939
+"""
+
+from textual.app import App
+from textual.containers import Container
+from textual.widgets import Static, Header, Footer, Button
+
+
+class NonFocusParent(Static):
+    def compose(self):
+        yield Button("Do Not Press")
+        yield Static("Test")
+        yield Button("Really Do Not Press")
+
+
+class FocusRemovalTester(App[None]):
+
+    BINDINGS = [("a", "add_widget", "Add Widget"), ("d", "del_widget", "Delete Widget")]
+
+    def compose(self):
+        yield Header()
+        yield Container()
+        yield Footer()
+
+    def action_add_widget(self):
+        self.query_one(Container).mount(NonFocusParent())
+
+    def action_del_widget(self):
+        candidates = self.query(NonFocusParent)
+        if candidates:
+            candidates.last().remove()
+
+
+if __name__ == "__main__":
+    FocusRemovalTester().run()