HamApps Support (VK3AMA)
I can now confirm the broken JTAlert behaviour (minimize and close buttons disabled) under the 1903 update of Windows 10.
At this time, it is unknown if a fix will be possible. This update has broken something within the quite complex (and convoluted) code needed to display the menus and Band-Activity in the titlebar.
It is still possible to close JTAlert, there are several methods available...
If you don't want to use one of these alternatives, the only solution is to install the "Alternate Layout" version of JTAlert.
I have confirmed the "Alternate Layout" build is working with Win10 1903.
de Laurie VK3AMA