Improve tests.

tests/snapshot_tests/snapshot_apps/programmatic_scrollbar_gutter_change.py

@@ -0,0 +1,29 @@
+from textual.app import App
+from textual.containers import Grid
+from textual.widgets import Label
+
+
+class ProgrammaticScrollbarGutterChange(App[None]):
+    CSS = """
+    Grid { grid-size: 2 2; scrollbar-size: 5 5; }
+    Label { width: 100%; height: 100%; background: red; }
+    """
+
+    def compose(self):
+        yield Grid(
+            Label("one"),
+            Label("two"),
+            Label("three"),
+            Label("four"),
+        )
+
+    def on_key(self, event):
+        if event.key == "s":
+            self.query_one(Grid).styles.scrollbar_gutter = "stable"
+
+
+app = ProgrammaticScrollbarGutterChange()
+
+
+if __name__ == "__main__":
+    app().run()

tests/snapshot_tests/test_snapshots.py

@@ -179,6 +179,16 @@ def test_nested_auto_heights(snap_compare):
     assert snap_compare("snapshot_apps/nested_auto_heights.py", press=["1", "2", "_"])
 
 
+def test_programmatic_scrollbar_gutter_change(snap_compare):
+    """Regression test for #1607 https://github.com/Textualize/textual/issues/1607
+
+    See also tests/css/test_programmatic_style_changes.py for other related regression tests.
+    """
+    assert snap_compare(
+        "snapshot_apps/programmatic_scrollbar_gutter_change.py", press=["s"]
+    )
+
+
 # --- Other ---