Fix regression from #6014 (#6018)

app/javascript/mastodon/features/ui/components/columns_area.js

@@ -27,8 +27,6 @@ const componentMap = {
   'LIST': ListTimeline,
 };
 
-const isRtlLayout = document.getElementsByTagName('body')[0].classList.contains('rtl');
-
 @component => injectIntl(component, { withRef: true })
 export default class ColumnsArea extends ImmutablePureComponent {
 
@@ -55,7 +53,10 @@ export default class ColumnsArea extends ImmutablePureComponent {
     if (!this.props.singleColumn) {
       this.node.addEventListener('wheel', this.handleWheel,  detectPassiveEvents.hasSupport ? { passive: true } : false);
     }
-    this.lastIndex = getIndex(this.context.router.history.location.pathname);
+
+    this.lastIndex   = getIndex(this.context.router.history.location.pathname);
+    this.isRtlLayout = document.getElementsByTagName('body')[0].classList.contains('rtl');
+
     this.setState({ shouldAnimate: true });
   }
 
@@ -81,7 +82,7 @@ export default class ColumnsArea extends ImmutablePureComponent {
 
   handleChildrenContentChange() {
     if (!this.props.singleColumn) {
-      const modifier = isRtlLayout ? -1 : 1;
+      const modifier = this.isRtlLayout ? -1 : 1;
       this._interruptScrollAnimation = scrollRight(this.node, (this.node.scrollWidth - window.innerWidth) * modifier);
     }
   }