I'm really happy to see so many design options being moved to the customizer. After all, it only makes sense to see the design while modifying it, right? However, there is one issue that has always bugged me about the customizer window -- it pushes the theme aside rather than overlapping it. When working on responsive themes, this leaves us looking at the responsive menu button, and widgets situated below the content. There is no sidebar, so styling the sidebar means having to collapse the customizer window to see any changes. I guess it's not really a big issue, but it just seems to make more sense if the customizer panel would overlap the theme.