[Glitch] Fix “Back” button sometimes redirecting out of Mastodon

Port a442a1d1c6 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/features/ui/index.jsx

@@ -528,11 +528,12 @@ class UI extends Component {
   };
 
   handleHotkeyBack = () => {
-    // if history is exhausted, or we would leave mastodon, just go to root.
-    if (window.history.state) {
-      this.props.history.goBack();
+    const { history } = this.props;
+
+    if (history.location?.state?.fromMastodon) {
+      history.goBack();
     } else {
-      this.props.history.push('/');
+      history.push('/');
     }
   };