Category menu shows default scrollbar

[Kenny2github]

Expected Behavior

The category menu should have no scrollbar, except one that stays invisible until scrolling actually happens.

Actual Behavior

The default scrollbar appears:

Steps to Reproduce

Open https://llk.github.io/scratch-gui/develop

Operating System and Browser

Windows 10 Chrome 61 - will try to repro on other OSes to see if it's OS-/browser-specific