MR image diff swipe view: Disable user-select on drag

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/59942

app/assets/javascripts/vue_shared/components/diff_viewer/viewers/image_diff/onion_skin_viewer.vue

@@ -102,7 +102,7 @@ export default {
       :style="{
         width: onionMaxPixelWidth,
         height: onionMaxPixelHeight,
-        'user-select': dragging === true ? 'none' : '',
+        'user-select': dragging ? 'none' : null,
       }"
       class="onion-skin-frame"
     >

app/assets/javascripts/vue_shared/components/diff_viewer/viewers/image_diff/swipe_viewer.vue

@@ -68,12 +68,10 @@ export default {
     },
     startDrag() {
       this.dragging = true;
-      document.body.style.userSelect = 'none';
       document.body.addEventListener('mousemove', this.dragMove);
     },
     stopDrag() {
       this.dragging = false;
-      document.body.style.userSelect = '';
       document.body.removeEventListener('mousemove', this.dragMove);
     },
     prepareSwipe() {
@@ -104,7 +102,13 @@ export default {
 
 <template>
   <div class="swipe view">
-    <div ref="swipeFrame" class="swipe-frame">
+    <div
+      ref="swipeFrame"
+      :style="{
+        'user-select': dragging ? 'none' : null,
+      }"
+      class="swipe-frame"
+    >
       <image-viewer
         key="swipeOldImg"
         ref="swipeOldImg"