Fixed dropdown menu max height styling such that it isn't overwritten by the DataTable styling when it is added to the layout. Added test for checking this as well

Author: aGitForEveryone

components/dash-core-components/src/components/css/Dropdown.css

@@ -1,7 +1,8 @@
 .dash-dropdown .Select-menu-outer {
     z-index: 1000;
+    max-height: none;
 }
 
-.dash-dropdown .Select-menu, .Select-menu-outer {
+.dash-dropdown .Select-menu {
     max-height: none;
 }

components/dash-core-components/tests/integration/dropdown/test_visibility.py

@@ -50,7 +50,10 @@ def test_ddvi001_fixed_table(dash_duo):
 def test_ddvi002_maxHeight(dash_duo):
     app = Dash(__name__)
     app.layout = Div(
-        [Dropdown([str(i) for i in range(100)], "1", id="dropdown", maxHeight=800)]
+        [
+            DataTable(),  # ensure datatable css does not override maxHeight #2529
+            Dropdown([str(i) for i in range(100)], "1", id="dropdown", maxHeight=800),
+        ]
     )
 
     dash_duo.start_server(app)