[DiapDealer]

There's probably some negative margins or 'pre' tags/css that causes text to run off the screen. Hence the mistaken notion that there's been some sort of limitation being reached. Either that, or there's unsupported fixed layout stuff going on.

Older versions of Sigil might have run into a 10Mb webengine limitation, but I have no idea how it might have manifested, and that's been fixed for quite a few releases, now.