make watchers instant

tests/test_reactive.py

@@ -0,0 +1,26 @@
+from textual.app import App, ComposeResult
+from textual.reactive import reactive
+
+
+class WatchApp(App):
+
+    count = reactive(0, init=False)
+
+    test_count = 0
+
+    def watch_count(self, value: int) -> None:
+        self.test_count = value
+
+
+async def test_watch():
+    """Test that changes to a watched reactive attribute happen immediately."""
+    app = WatchApp()
+    async with app.run_test():
+        app.count += 1
+        assert app.test_count == 1
+        app.count += 1
+        assert app.test_count == 2
+        app.count -= 1
+        assert app.test_count == 1
+        app.count -= 1
+        assert app.test_count == 0