fix: avoid nested select widget z-index conflicts (#2990)

packages/netlify-cms-ui-default/src/styles.js

@@ -352,7 +352,7 @@ const reactSelectStyles = {
       : 'transparent',
     paddingLeft: '22px',
   }),
-  menu: styles => ({ ...styles, right: 0, zIndex: 2 }),
+  menu: styles => ({ ...styles, right: 0, zIndex: 300 }),
   container: styles => ({ ...styles, padding: '0 !important' }),
   indicatorSeparator: (styles, state) =>
     state.hasValue && state.selectProps.isClearable

packages/netlify-cms-widget-markdown/src/MarkdownControl/VisualEditor.js

@@ -28,6 +28,7 @@ const visualEditorStyles = `
   padding: 0;
   display: flex;
   flex-direction: column;
+  z-index: 100;
 `;
 
 const InsertionPoint = styled.div`