A few AWT mixing tests in test/jdk/java/awt/Mixing/AWT_Mixing/ use the class FrameBorderCounter class to calculate the size of title bar, namely:

HierarchyBoundsListenerMixingTest.java
MixingPanelsResizing.java
OverlappingTestBase.java

The FrameBorderCounter class is launched as a child process, it prints
int shift = entered.x - location.x - size.width - 1;
to the standard output which is then parsed in the parent process.

The child process is not needed, the logic can be executed in-process, and the result can be returned directly as a result of a method called.