Invisible scrollbar presents usability issue

Steps to reproduce:

• Set macOS preferences > general > show scroll bars > "when scrolling" (system default)
• Using Chrome in macOS
• Navigate to about.gitlab.com
• In the footer, click on "open in Web IDE"

Observed:

• The left pane's scrollbar is missing.

Expected:

• Visible scrollbar for people who aren't power-users.

Suggested fix:

• Switch .ide-tree-body from overflow: auto to overflow: scroll and then apply some extra css : https://davidwalsh.name/osx-overflow

Other notes:

• While it can be good to use system defaults, this might at times present usability issues.
• There isn't a known css-based fix for firefox. There are JS alternatives.

Issue source:

I was on a zoom call earlier explaining how to use the Web IDE's left pane.. The user could not see a scrollbar and so couldn't figure out how to open other files. I finally found a workaround using the arrow keys to scroll.

Per a slack conversation, pinging @mvanremmerden.