The extension has been fully rewritten to provide a more compact and flexible UI as well as the following new features and bug fixes:

• Grid definition is more bootstrap compatible (if you want different ways to define a grid feel free to open an issue).
• All length can be express in any CSS units and calc() is also allowed (be careful with relative units as they are only resolved against the body sizes).
• Grids and rulers are now fully persistent on a per domain basis, they'll appear each time you'll open the devtools (and disappear each time you'll close them).
• Remove keyboard shortcuts as they were too buggy and not that useful (open an issue if you want them back).
• Improve UI accessibility (It can now be fully operated with the keyboard).