Playground: Do not black out content while editing the grammar

[Lotes]

For demonstration purposes it is annoying that the content disappears while the grammar is edited, because one want to orient along the desired content. So, you have to remember what to parse or copy the content over to the grammar.

One solution could be an opacity of < 100 % or a disabling or a grey-out-effect, so that the content is still readable and maybe also scrollable(?).

Manually test this by fiddling on a grammar with small and with big content to look who suites best.