ui: fix a small display issue

packages/ui-default/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@hydrooj/ui-default",
-  "version": "4.26.18",
+  "version": "4.26.19",
   "author": "undefined <i@undefined.moe>",
   "license": "AGPL-3.0",
   "main": "hydro.js",

packages/ui-default/pages/problem_detail.page.jsx

@@ -49,6 +49,7 @@ class ProblemPageExtender {
     $('.main > .row').hide();
     $('.footer').hide();
     $(window).scrollTop(0);
+    window.document.body.style.overflow = 'hidden';
 
     this.inProgress = false;
     this.isExtended = true;
@@ -83,6 +84,7 @@ class ProblemPageExtender {
 
     this.$scratchpadContainer.hide();
     this.$content.transition({ opacity: 1 }, { duration: 100 });
+    window.document.body.style.overflow = 'scroll';
 
     this.inProgress = false;
     this.isExtended = false;