Discussion at AWT mailing list - http://mail.openjdk.java.net/pipermail/awt-dev/2017-March/012661.html

Steps to reproduce it with attached WindowTest.java, maybe you need 2-3 attempts:

(0. start the demo program)
1. click on the button in the first window ; a second window opens
2. click on the button in the second dialog ; a third windows open
3. move the third window via mouse and click on the label
4. wait 9 seconds ; a fourth windows is opened (the fourth windows should be opened behind the third)
5. try to close the fourth
-> EDT is blocked

The relevant thread stacks are:

"AWT-EventQueue-0" #13 prio=6 os_prio=31 tid=0x00007fa86ea7e000 nid=0xd203 runnable [0x0000700010632000]
   java.lang.Thread.State: RUNNABLE
    at sun.lwawt.macosx.CWrapper$NSWindow.isZoomed(java.desktop@9-ea/Native Method)
    at sun.lwawt.macosx.CPlatformWindow.isMaximized(java.desktop@9-ea/CPlatformWindow.java:509)
    at sun.lwawt.macosx.CPlatformWindow.setVisible(java.desktop@9-ea/CPlatformWindow.java:558)
    at sun.lwawt.LWWindowPeer.setVisibleImpl(java.desktop@9-ea/LWWindowPeer.java:253)
    at sun.lwawt.LWComponentPeer.setVisible(java.desktop@9-ea/LWComponentPeer.java:763)
    at java.awt.Component.hide(java.desktop@9-ea/Component.java:1705)
    - locked <0x00000007a48a2c20> (a java.awt.Component$AWTTreeLock)
    at java.awt.Window.hide(java.desktop@9-ea/Window.java:1118)
    at java.awt.Dialog.hide(java.desktop@9-ea/Dialog.java:1152)
    at java.awt.Window.hide(java.desktop@9-ea/Window.java:1110)
    - locked <0x00000007a802e218> (a java.util.Vector)
    at java.awt.Dialog.hide(java.desktop@9-ea/Dialog.java:1152)
    at java.awt.Component.show(java.desktop@9-ea/Component.java:1670)
    at java.awt.Component.setVisible(java.desktop@9-ea/Component.java:1615)
    at java.awt.Window.setVisible(java.desktop@9-ea/Window.java:1017)
    at java.awt.Dialog.setVisible(java.desktop@9-ea/Dialog.java:1005)
    
"AppKit Thread" #11 daemon prio=5 os_prio=31 tid=0x00007fa87076b000 nid=0x307 waiting for monitor entry [0x00007fff5e87e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at java.awt.Window.getOwnedWindows_NoClientCode(java.desktop@9-ea/Window.java:1478)
    - waiting to lock <0x00000007a802e218> (a java.util.Vector)
    at java.awt.Window$1.getOwnedWindows(java.desktop@9-ea/Window.java:4127)
    at sun.lwawt.macosx.CPlatformWindow.orderAboveSiblingsImpl(java.desktop@9-ea/CPlatformWindow.java:1153)
    at sun.lwawt.macosx.CPlatformWindow.orderAboveSiblings(java.desktop@9-ea/CPlatformWindow.java:1118)
    at sun.lwawt.macosx.CPlatformWindow.windowDidBecomeMain(java.desktop@9-ea/CPlatformWindow.java:1182)