Cannot "pan" editor (scroll horizontally) via mousewheel or trackpad
I can scroll vertically with a trackpad gesture (or via the mousewheel), but you're not allowing the same convenience for horizontal panning. I'm not sure why. On many apps, including PhotoShop CC, which it's obvious that you're imitating, holding shift while scrolling pans the workspace. Very convenient.
With widget panels open on both sides, the edit window is quite a bit narrower than the Web site I'm designing, so I spend more time than I'd like acquiring that narrow scrollbar target and dragging it from side to side. It's an unsatisfying user experience.
Moreover, unlike most scrollbars, clicking in the track outside the scrolling "thumb" does not jump by a screenful, but by some tiny increment. Also inconvenient and unsatisfying.