For the first time in 3.5 years, we had soy.windows.Window running on OSX. Just the basics; opening, (re)setting the title, resizing - but it worked. This is thanks to GDK, the windowing and event backend to GTK+, running on Mac OSX via native Quartz backend (not X11).
Shortly after we moved the window code to a background thread and it broke. The code runs fine on GNU/Linux and Windows, but on OSX all we get are white rectangles where the windows would normally be rendered. The rectangles even resize properly, and window events are getting processed by our code, just nothing rendered inside (not even the window title or buttons). I filed this bug and hope the problem to be solved quickly.
The problem could be as simple as a missing NSGraphicsContext.flushGraphics call (which is only needed when rendering is done in a non-main thread). If the bug hasn't been fixed by our next release I'll look into patching this myself.