Horizontal scroll bar under tabs can disappear

[kena0ki]

Bug Description:

Unlike VS Code, the horizontal scroll bar under the tabs can disappear when opening many files.
(Maybe this is not clearly a bug, but I think this is a bug in terms of usability.)

Steps to Reproduce:

1. open many files
2. scroll to right
3. scroll bar disappears

Theia (Gitpod):

VS Code:

Additional Information

• Operating System: any (not related)
• Theia Version: 1.9.0

[colin-grant-work]

This is particularly problematic when the central panel is split. Then one scroll bar easily extends into (and behind) the other side's tabbar.