I happen 2 use Windows –I haven’t tried Linux or Macintosh– and it happens in different machines in every chromium or not-chromium-based browsers (from edge legacy to chrome/brave/vivaldi/etc).
The issue follows as this: in order to add more horizontal content in an horizontal-scroll layouter once its viewport is covered with the first contents, you have to scroll in order to access the other layouter items, but it’s impossible to do so from the editing mode; only the preview mode and the published site allow to scroll (by dragging the overflow bar or wheelscrolling while holding shift) but that serves no purpose since it needs to be achievable from the Editor per se in order to be… well, editable
Additionally, the horizontal scroll by mousewheel works but –as said in the previous paragraph– only while holding SHIFT key, which is not intuitive or user-friendly, and so far there’s no way to ‘hack’ it into being automatic through Velo nor custom elements ://