Running on my MBPro I have my GDK scale set to 2 in the control panel.  When I run a JavaFX app with GTK2 it runs fine (though unscaled), but when I run GTK3 it behaves as if GTK is trying to do scaling for us, but we didn't notice.  In particular:

- The window appears in the upper left quadrant of the screen - as if the screen bounds were reported to us scaled by half, which is what you'd expect if they are trying to handle the GDK_SCALE=2 for us.

- Ensemble lays itself out in the lower left 1/4 of the window, which is what you might expect if we are told the window is half the width and height that we'd asked for.

I am adding HiDPI support in and discovered these issues when I tested with GTK3.  They need to be fixed, but they are independent of what is going on with my HiDPI fixes (in particular, the issues appear identical before and after that fix) so I am filing this bug to investigate after I get the base HiDPI support in.