Add regression test.

tests/css/test_programmatic_style_changes.py

@@ -0,0 +1,64 @@
+import pytest
+
+from textual.app import App
+from textual.containers import Grid
+from textual.screen import Screen
+from textual.widgets import Label
+
+
+updates = 0
+
+
+class _Label(Label):
+    """Label widget that keeps track of its own updates."""
+
+    def refresh(self, *args, **kwargs):
+        global updates
+        updates += 1
+        return super().refresh(*args, **kwargs)
+
+
+@pytest.mark.parametrize(
+    "style, value",
+    [
+        ("grid_size_rows", 2),
+        ("grid_size_columns", 2),
+        ("grid_gutter_vertical", 2),
+        ("grid_gutter_horizontal", 1),
+        ("grid_rows", "1fr 3fr"),
+        ("grid_columns", "1fr 3fr"),
+        ("overflow_x", "scroll"),
+        ("overflow_y", "scroll"),
+        ("scrollbar_gutter", "stable"),
+        ("align_horizontal", "right"),
+        ("align_vertical", "bottom"),
+        ("align", ("right", "bottom")),
+    ],
+)
+def test_programmatic_style_change_refreshes_children_layout(style: str, value):
+    """Regression test for #1607 https://github.com/Textualize/textual/issues/1607
+
+    Some programmatic style changes to a widget were not updating the layout of the
+    children widgets, which seemed to be happening when the style change did not affect
+    the size of the widget but did affect the layout of the children.
+    """
+
+    global updates
+
+    app = App()
+    app.DEFAULT_CSS = "Grid { grid-size: 1 1; }"
+    app._set_active()
+    app.push_screen(Screen())
+
+    grid = Grid(
+        _Label("one"),
+        _Label("two"),
+        _Label("three"),
+        _Label("four"),
+    )
+    app.screen._add_children(grid)
+
+    update_count = updates
+    setattr(grid.styles, style, value)
+    print(updates, update_count)
+    assert updates > update_count