[Glitch] Fix scroll position resetting when opening media modals in web UI

Port ad91a0578b to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/containers/mastodon.js

@@ -49,6 +49,10 @@ export default class Mastodon extends React.PureComponent {
     }
   }
 
+  shouldUpdateScroll (_, { location }) {
+    return !(location.state && location.state.mastodonModalOpen);
+  }
+
   render () {
     const { locale } = this.props;
 
@@ -57,7 +61,7 @@ export default class Mastodon extends React.PureComponent {
         <Provider store={store}>
           <ErrorBoundary>
             <BrowserRouter basename='/web'>
-              <ScrollContext>
+              <ScrollContext shouldUpdateScroll={this.shouldUpdateScroll}>
                 <Route path='/' component={UI} />
               </ScrollContext>
             </BrowserRouter>