Vertical scrollbar not visible in editor sidebar when the height of the sidebar exceeds the screen

**Describe the bug**
On a white background the scrollbar in the sidebar in the editor is not visible (white thumb on white background). The main scrollbar thumb of the editor is gray and visible. After validation and opening the validation panel you can not see all the info. You can only use your mouse wheel to scroll down.

**To Reproduce**
1. Create or edit a metadata record and click on the Validation button.
2. Open the validation panel in the sidebar
3. Look at the end of the panel and you see there is more info available but can only scroll down with the mouse wheel

**Expected behavior**
A visible scroll thumb to scroll down.

**Screenshots**

**Log file**
Not applicable

**Desktop (please complete the following information):**
- Only in Chrome the thumb is not visible, in Firefox the thumb is available.
- GeoNetwork 3.8.3
- Tomcat 8.5 with Java 8

**Additional context**
By disabling web-ui/src/main/resources/catalog/style/gn_editor.less the . .scrollbar-white(); line, the thumb is visible too in Chrome.

@media (min-width: @screen-md-min) {
.scrollbar-white() {
&::-webkit-scrollbar-track, &::-webkit-scrollbar-thumb {
-webkit-box-shadow: inset 0 0 6px #fff;
}
&::-webkit-scrollbar-thumb:window-inactive {
background: #fff;
}
}
.gn-editor-sidebar {
margin-top: 0;
position: fixed;
right: 0;
overflow: auto;
max-height: 100%;
padding-bottom: 110px;
**// Scrollbar not visible in Chrome
//.scrollbar-white();**
}
}