Add regression test for #1616.

tests/test_overflow_change.py

@@ -0,0 +1,38 @@
+"""Regression test for #1616 https://github.com/Textualize/textual/issues/1616"""
+import pytest
+
+
+from textual.app import App
+from textual.containers import Vertical
+
+
+@pytest.mark.xfail("Needs #1610 so that overflow changes trigger layout recomputation.")
+async def test_overflow_change_updates_virtual_size_appropriately():
+    class MyApp(App):
+        def compose(self):
+            yield Vertical()
+
+    app = MyApp()
+
+    async with app.run_test() as pilot:
+        vertical = app.query_one(Vertical)
+
+        height = vertical.virtual_size.height
+
+        vertical.styles.overflow_x = "scroll"
+        await pilot.pause()  # Let changes propagate.
+        assert vertical.virtual_size.height < height
+
+        vertical.styles.overflow_x = "hidden"
+        await pilot.pause()
+        assert vertical.virtual_size.height == height
+
+        width = vertical.virtual_size.width
+
+        vertical.styles.overflow_y = "scroll"
+        await pilot.pause()
+        assert vertical.virtual_size.width < width
+
+        vertical.styles.overflow_y = "hidden"
+        await pilot.pause()
+        assert vertical.virtual_size.width == width