In case Xinerama is on the Device.getDisplayMode() cannot get current display mode from native platform for secondary screens because it is queried using old xrandr API. The new xrandr API should be used in this case.
JDK-8159953Make GraphicsDevice#getDisplayMode() to return correct mode for Xinerama multi-screen
JDK-8173890Dual-screen linux: still no current mode in getDisplayModes()
JDK-8022810Cannot list all the available display modes on Ubuntu linux in case of two screen devices