uiWindowSetChild(uiArea) does not fill the window on GTK+

Hello, either I am doing something wrong, or drawing to an area no longer works on linux.

I created [this](https://gist.github.com/FlyingGaz/e361978716e8e607a05de846ad6387a8) example which is supposed to draw a red triangle.
When running the example with the current version of libui, I only get an empty window.
If I switch to [this](https://github.com/andlabs/libui/tree/156c3584f706753e892be77338d1b9c2cb669c5b) older version of libui, I get the expected result.

I'm running Archlinux with the current version of GTK3 (3.20.9-1).