I've dabbled with this more than once over the years I've been using Roundcube, but always end up reverting to the default config. TinyMCE's support for this "feature" is rudimentary at best - they don't recommend or support it, and the implementation is spotty. I've always experienced odd composing bugs whenever I tried this, so I gave up. Turns out that my users have learned to accept it now, and I no longer get complaints about it.