Fix overflow issue in WelcomeScreen

Summary: Noticed that the welcome screen started bleeding in the sidebar when the screen is too small. Unlike the plugin container it lacked an overflow: hidden.

Reviewed By: timur-valiev, aigoncharov

Differential Revision: D32987160

fbshipit-source-id: be324247cdb4abfeec12c40624d65f9fa1e86eff

desktop/flipper-ui-core/src/sandy-chrome/SandyApp.tsx

@@ -224,6 +224,7 @@ const outOfContentsContainer = (
 const MainContainer = styled(Layout.Container)({
   background: theme.backgroundWash,
   padding: `${theme.space.large}px ${theme.space.large}px ${theme.space.large}px 0`,
+  overflow: 'hidden',
 });
 
 const RootElement = styled.div({

desktop/flipper-ui-core/src/sandy-chrome/WelcomeScreen.tsx

@@ -129,6 +129,7 @@ export function WelcomeScreenStaticView() {
       center
       style={{
         justifyContent: 'center',
+        overflow: 'hidden',
       }}
       pad
       grow>