Set webkit-overflow-scrolling to auto for children of body.

CHANGELOG.md

@@ -126,6 +126,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - API: all unknown routing will be handled with 404 Not Found
   - Add docs for request profiling
   - Delete dynamic environments
+  - Fix buggy iOS tooltip layering behavior.
   - Make guests unable to view MRs on private projects
 
 ## 8.12.7

app/assets/stylesheets/framework/files.scss

@@ -167,7 +167,6 @@
      */
     &.code {
       padding: 0;
-      -webkit-overflow-scrolling: auto; // See https://gitlab.com/gitlab-org/gitlab-ce/issues/13987
     }
   }
 }

app/assets/stylesheets/framework/layout.scss

@@ -27,3 +27,15 @@ body {
 .container-limited {
   max-width: $fixed-layout-width;
 }
+
+
+/* The following prevents side effects related to iOS Safari's implementation of -webkit-overflow-scrolling: touch,
+which is applied to the body by jquery.nicescroling plugin to force hardware acceleration for momentum scrolling. Side
+effects are commonly related to inconsisent z-index behavior (e.g. tooltips). By applying the following to direct children
+of the body element here, we negate cascading side effects but allow momentum scrolling to be applied to the body  */
+
+.navbar,
+.page-gutter,
+.page-with-sidebar {
+  -webkit-overflow-scrolling: auto;
+}

app/assets/stylesheets/pages/diff.scss

@@ -52,7 +52,6 @@
     background: #fff;
     color: #333;
     border-radius: 0 0 3px 3px;
-    -webkit-overflow-scrolling: auto;
 
     .unfold {
       cursor: pointer;