ADDITIONAL SYSTEM INFORMATION :
macOS 10.15.7
JavaFX 15.0.1
openjdk15/jdk-15.0.1+9

A DESCRIPTION OF THE PROBLEM :
This issue seems to be the same as described in https://bugs.openjdk.java.net/browse/JDK-8214209

You can find more context, steps to reproduce, and analysis on this ticket as well: https://github.com/lwouis/alt-tab-macos/issues/822#issuecomment-778902109

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
* Run the AltTab app (download .app here: https://alt-tab-macos.netlify.app/)
* Run the MediathekView app: https://github.com/mediathekview/MediathekView.git (download the pre-built mac app, or git clone + run it)
* In MediathekView, click the "Merkliste verwalten" button (see screenshot here: https://github.com/lwouis/alt-tab-macos/issues/822#issuecomment-778902109)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
A window opens and the app runs normally
ACTUAL -
The app freezes indefinitely (deadlock)

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You can run/debug the source code of MediathekView here: https://github.com/mediathekview/MediathekView I did it and found that it deadlocks in JavaFX code. More analysis from me: https://github.com/lwouis/alt-tab-macos/issues/822#issuecomment-778916280
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CUSTOMER SUBMITTED WORKAROUND :
The app works correctly if AltTab is not running. I'm the author of AltTab, and I've seen similar issues in Java apps. I've seen it in Jetbrains product for instance, and they are not using JavaFX I imagine. That could imply that the issue is lower in the stack (e.g. in the AWT layers).

In AltTab, I can prevent the issue triggering in MediathekView by removing 1 line of code, so it seems to be the issue. That line is a call to `AXObserverAddNotification` to get notified of the Accessibility event called `kAXFocusedUIElementChangedNotification`. Observing that event creates an issue in Java apps that results in a deadlock apparently.

FREQUENCY : always