Comment 7 for bug 2017323

I've tracked down the origin of the 16384 limit. It's a combination of:

 * Xorg treating all monitors as a single framebuffer; and
 * The modesetting driver getting the maximum dimensions from the kernel via drmModeResPtr; and
 * The Intel kernel graphics driver limiting framebuffer sizes to 16384. This Intel limitation is unavoidable if you want to use the laptop screen as part of your desktop.

You can also see it in the log:

  modeset(0): Allocate new frame buffer 16384x...

So I have a couple of suggestions for getting around it:

 * Use a lower scaling factor or the default of 100%; or
 * Remove one monitor from the system