Add "overlay" sidebar position to MaterDetail

Summary: Project: https://docs.google.com/document/d/1x-1ShlxXCM2od9JMO6PP14Cuqq0OrVe1Qd7FUwTos9A/edit

Reviewed By: ivanmisuno

Differential Revision: D44706158

fbshipit-source-id: 16c70a7c9a414a6c32d690eb05ae5b08818dd400

desktop/flipper-plugin/src/ui/MasterDetail.tsx

@@ -41,7 +41,7 @@ type MasterDetailProps<T> = {
    * 'bottom': show a resizable pane to the bottom
    * 'none': don't show details at all
    */
-  sidebarPosition?: 'bottom' | 'right' | 'main' | 'none';
+  sidebarPosition?: 'bottom' | 'right' | 'main' | 'overlay' | 'none';
   /**
    * Component that accepts a 'record' prop that is used to render details.
    * If none is provided, a standard `DataInspector` component will be used to display the entire record.
@@ -245,6 +245,13 @@ export function MasterDetail<T extends object>({
           {sidebar}
         </Layout.Bottom>
       );
+    case 'overlay':
+      return (
+        <Layout.Container grow style={{position: 'relative'}}>
+          {table}
+          {sidebar}
+        </Layout.Container>
+      );
     case 'none':
       return table;
   }