It’s observed that, getting the component count of the JDesktopPane returns one extra component than expected, when run with -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel.

The spec of - https://download.java.net/java/early_access/jdk18/docs/api/java.desktop/java/awt/Container.html#getComponentCount()
"Gets the number of components in this panel."

Generally the Container(JDesktopPane) created fresh one does not expect any components by default.
If the behavior of the JDesktopPane:getComponentCount() varies based on Look and feel then this should be stated explicitly in spec.

Please find the code snipped to demonstrate the problem

JDesktopPane jDesktopPane = new JDesktopPane();
if (jDesktopPane.getComponentCount() == 0) {
    System.out.println("Expected : no child components present");
} else {
    System.out.println("Unexpected : by default the jDesktopPane has components size " + jDesktopPane.getComponentCount() + " And the child component is " + jDesktopPane.getComponent(0));
}


Observation:
=====

/scratch/jdk-cache/17/34/jdk-17/bin/java -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel JDesktopPaneCompTests
Unexpected : by default the jDesktopPane has components size 1 And the child component is javax.swing.plaf.synth.SynthDesktopPaneUI$TaskBar

/scratch/jdk-cache/17/34/jdk-17/bin/java JDesktopPaneCompTests
Expected : no child components present