How about resizable accordions instead of tabs, like codepen? On the desktop, they be like the right-hand side stacked editors. On mobile, they pull over the content like a drawer covering the entire screen, open one pane at a time?
What does everyone think about having the left sidebar as the main nav (instead of on top), and a right side bar for page content nav (instead of on the left)? On mobile, it would either still be a top collapsible and hidden content nav... or a left and right drawer menu.