Merge branch '24851-collapsed-system-note-list-shadow-looks-weird' into 'master'

fixes non-retina shadow and browser zoom issue

## What does this MR do?
This MR closes #24851 and shadow issue for browser zoom 

## Are there points in the code the reviewer needs to double check?
NR

## Screenshots (if relevant)
**Ratina Display:**
![normal](/uploads/18fb0e321eff55cb89413745413cfe8a/normal.png)

**Non Retina**
![non-retina](/uploads/85ce8d3b22a025c251142bebe30c4401/non-retina.png)

**110% zoom**
![ratina](/uploads/bc0f2468ba780998e0e2b60a6d07e8db/ratina.png)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #24851

See merge request !7712

app/assets/stylesheets/pages/notes.scss

@@ -75,6 +75,8 @@ ul.notes {
         display: none;
         padding: 10px 0 0;
         cursor: pointer;
+        position: relative;
+        z-index: 2;
 
         &:hover {
           color: $gl-link-color;
@@ -118,11 +120,11 @@ ul.notes {
           &::after {
             content: '';
             width: 100%;
-            height: 20px;
+            height: 67px;
             position: absolute;
             left: 0;
-            bottom: 50px;
-            background: linear-gradient(rgba($gray-light, .3) 0, $white-light);
+            bottom: 0;
+            background: linear-gradient(rgba($gray-light, 0.1) -100px, $white-light 100%);
           }
 
           &.hide-shade {