So I had this problem earlier this week.
What you do is select 'scale outer' in the project settings for fullscreen in browser. This is what cocoonjs defaults to if if you use a letterbox view (as its unsupported by cocoonjs). However If you designed your game for widescreen you'll find that when previewing it in cocoonJS you'll get a white strip along the right edge of the screen.
So what you need to do is make the layout width larger than the window size width. My windows size was 900x1600. So I increased each of my layouts to 1200x1600. It will add the extra space to the right side of the screen. Which means if you have centered things based on your original layout size you will have to rejig things to your new center on all your layouts (yeah it was a pain). Then if you want to center the layout use the 'Scroll To' behavior on something that is in the horizontal center of your screen.
kapeesh?
Does anyone know another solution than this? For my game I have objects wrapping around the layout and this ruins the game mechanic since the wrap around will happen somewhere out in the scale outer. I have tried both wrap to layout and viewport but both cause problems.
I could go in and recode my own wraparound code but that would require a lot of effort this far into the games development.
Thanks