1 why do you disable contenteditable for desktops? what exact problems does it have?
It is terribly buggy on older browsers, so the textarea mode has to stay for the time being. And the contenteditable mode is a rather scary new bunch of hacks, and so far I haven’t felt safe pushing it on all desktop users, since they’ll be bitten by all the new bugs. I am planning to make it the default in the next major release, for modern browser, and start calling textarea mode ‘legacy’, but first I’ll need to be able to allocate the time needed to actually deal with the bug fallout that’ll produce.
2 why do you disable spellchecking for contenteditable?
For consistency. I’d be happy to expose this as an option, and document that it only works in contenteditable mode. A pull request to that effect would be welcome. My experiences with ProseMirror suggest that combining this kind of constant-redrawing display style with spell check works relatively well.
3 when we may have stable contenteditable for desktops?
Who knows. If everything goes well for me, maybe later this year, but with shakily funded work like this, I can’t make promises.