FIX: don't trigger header:hide-topic at the bottom on desktops

On mobile we trigger `header:hide-topic` at the very bottom of topics to switch the header contents back from small logo + topic info to large logo + user panels. 

Given that the `topic-progress` component is sometimes loaded on desktop - E.g composer is open or on narrow desktop screens - we need a guard to prevent this logic from firing on desktops.

app/assets/javascripts/discourse/components/topic-progress.js.es6

@@ -180,6 +180,10 @@ export default Ember.Component.extend({
     }
 
     // switch mobile scroll logo at the very bottom of topics
+
+    // not a mobile device so don't do anything
+    if (!this.site.mobileView) return;
+
     const isIOS = this.capabilities.isIOS,
       switchHeight = bodyHeight - offset - windowHeight,
       appEvents = getOwner(this).lookup("app-events:main");