Revisiting this thread again, because I seem to have fixed my issues with the Chrome Web Store.
What worked for me: remove the div element in which the canvas is enclosed in the index.html file. I know it says "The canvas must be inside a div called c2canvasdiv" in the file, but that doesn't appear to hold ground.
Then change the canvas tag to look like the following:
<canvas id="c2canvas" style="display: block; margin: 0px auto;">
Projects that are exported with fullscreen off have some unnecessary space at the bottom of the canvas. This change gets rid of it.
It also makes the canvas always be centered even if you resize it at runtime btw.
I didn't test these changes exhaustively, but it does indeed work for my project. But do this at your own risk of course, keep backups.
Another chrome web store related hint: if you make use of a plugin like Clay.io, which will communicate with an external server, you need to add an additional line to your manifest.
"content_security_policy":"script-src 'self' 'unsafe-eval' clay.io https://clay.io:846; object-src 'self'",
Of course the specified URLs are for the Clay.io plugin. Switch them with the ones you need.