Comment 34 for bug 1520969

Just a "wouldn't it be nice" moment:
https://github.com/AdaCore/gtkada/blob/master/testgtk/create_splittable.adb (like .c)
https://github.com/AdaCore/gtkada/blob/master/testgtk/create_splittable.ads (like .h)
which uses:
https://github.com/AdaCore/gtkada/blob/master/src/gtkada-multi_paned.adb
https://github.com/AdaCore/gtkada/blob/master/src/gtkada-multi_paned.ads

And gives the following attached screenshot (from the testgtk program).

It's actually more than we would need, because I think a single widget contains is going all those splits. It also has a non-opaque mode that would be a trivial answer to the "too many huge buffers with reflow will resize badly" concern. Of course I have never looked at Ada before today, so have no chance of porting, and don't even understand if we could achieve the rebalancing using that widget.