So, last April I disabled this feature, because I'd discovered that no matter how much I worked on fixing it across browsers, nested flexboxes have an undefined behavior.
Today, for some reason, I decided I wanted to see if I could do without the parent flexbox, and only use the child. So, I experimented on it, and added a dummy div in postarea, and made it absolutely positioned, but then I remembered the table cell itself could be relatively positioned, because of a Firefox bug or something, so I tweaked everything, and managed to get the child div to take the entire height of the cell, *without even resorting to absolute positioning*!
This is probably a Chrome bug or something, but from my tests (and I assure you, it was a totally random shot, not some hack I read from another website), setting the table's height to 100% (which doesn't have any impact) allows me to set it on its cells as well. Odd, eh..? And then I can also set height: 100% on the child div. It works, and allows me to turn the div into a flexbox, but unfortunately, the height: 100% trick didn't work on other browsers, and thus can't be used there, AFAIK.
Then I thought about it some more, and decided I didn't give a damn... I'm on Chrome, I can decide to 'improve' its layout if it supports it; other browsers will just behave as usual. I also removed the dummy div, and tried doing display: flex on the postarea, and it friggin' worked. I can't believe it, eh..?!! Anyway, considering that it's Chrome-only now, I'll spot bugs soon enough, so, I suppose, it's time to restore flexbox into Wedge, and enjoy the moment.
'course, as usual, if someone finds a truly cross-browser way to do it, feel free to point it out...!
This will be in my next commit, I think... Along with other things, hmm... Can't tell, it's top secret.