After a switch of JRE from J8 to J11 JComboBox instances in two different JPanels respectively (both in a JDesktopPane) don't display the popup menu when a respective containing JPanel is not in the focus when a click is happening. The first click brings focus into the respective JPanel. After that a single click on JComboBox causes the menu to pop up.

This changed when I switched the JRE from 8 to 11.

From the end user perspective it looks like a mess, because most components on the panel don't act like this and correctly do mouse click stuff regardless of the parent panel focus status.

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It was this commit for this ticket: JInternalFrame generates UngrabEvent when unselected, which causes JComboBox popup to hide.