[winswitch] Popup not shown

[Wolfram Humann]

Hi,
(server: xpra 3.0.3 on RHEL, client: xpra 3.0.3 on Win10)
I have a Java/Swing/Batik based tool that controls some electronic
hardware. When I click on certain components, a popup window should appear
-- but it does not. I noticed that behavior before updating xpra to 3.0.3
(some 3.0.1 or 3.0.2 versions) and hoped the update might fix it, but it
doesn't.
The popup does not appear when it should but an icon on the windows taskbar
appears, indicating that there should be a popup. I minimized all windows
to make sure that the popup is not hidden behind one of them.
The popup appears when I use cygwin/xwin connection to the RHEL workstation
instead of xpra.
Any idea?

(as a sidenote, the main-window of the tool sometimes [not always] is just
black when started. This can be fixed by slightly resizing the tool's
window)

Regards
Wolfram