Undocked panels jump around desktops (MacOS) when second display is used

[Polarbear-j]

System: Apple, two displays
MacOS El Capitan, 10.11.1
System Preferences -> Mission Control -> "Displays have separate Spaces" ON
This seems to be the default in 10.11 and causes a secondary display to run a separate desktop (Spaces in Apple-speak).
Can be verified with the F4-key (Mission Control) to show a separate desktop number, and both screens having a menu bar, one of them dimmed.

Identification: JOSM/1.5 (9069 en) Mac OS X 10.11.1
Java version: 1.8.0_66, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM

tested on a blank test user, no plugins installed.

How to reproduce:

Open JOSM. Load some data.
Undock a panel, e.g. Layers.
Move this panel to the second display.
Click the main JOSM window on the first display to get focus.
Click the undocked panel on the title bar. The panel quickly appears on the first display and reappears on the second.
Click again the main JOSM window on the first display to get focus.
Click the undocked panel in the content area. It jumps to the first display and stays there.

When "Displays have separate Spaces" is OFF, the two displays form one desktop, and the bug is not present. I was migrating from 10.8 to 10.11, so I don't know how the behaviour changed in the versions in between.

[anonymous]

It appears that the window jumps from the secondary to the main desktop to the same numerical position. So apparently it needs, beside the xy-coordinates, to memorise its desktop number as well.

[Stereo]

Ticket #13109 has been marked as a duplicate of this ticket.

[Stereo]

Can't reproduce in my Java 14 builds; assuming this was a Java 8 bug.