[Gorm][WinUX theme] bug in hide functionality

[gcasa]

The "Hide" functionality of Gorm is broken in a subtle way when using the WinUXTheme on Windows XP

steps to reproduce

freshly install GNUstep on Windows XP using
http://ftpmain.gnustep.org/pub/gnustep/binaries/windows/gnustep-system-0.24.2-setup.exe
http://ftpmain.gnustep.org/pub/gnustep/binaries/windows/gnustep-core-0.24.3-setup.exe
http://ftpmain.gnustep.org/pub/gnustep/binaries/windows/gnustep-devel-1.0.0-setup.exe
set the GNUstep theme to WinUXTheme during installation
install Gorm from SVN
launch Gorm (openapp Gorm)
choose the "Document > New Application" menu
-> the windows "Untitled-1", "My Window" and "Main Menu" appear. See attached Gorm-hide_bug-1.png
choose the "Gorm > Hide" menu
-> the windows "Untitled-1", "My Window" and "Main Menu" hide into a single button "Untitled-1" in the Windows XP taskbar. See attached Gorm-hide_bug-2.png
click on the button "Untitled-1" in the Windows XP taskbar
-> only the window "Untitled-1" is restored, but a button "My Window" appears in the Windows XP taskbar. See attached Gorm-hide_bug-3.png
-> clicking on that button "My Window" toggles the button but the window "My Window" doesn't appear again. See attached Gorm-hide_bug-4.png
-> the window "Main Menu" can be opened again by double clicking on "NSMenu" in "Untitled-1"
-> the window "My Window" can't be opened again by double clicking on "My Window" in "Untitled-1"
-> this doesn't happen always as it seems but in about 2/3 of all tries.
-> when you get two buttons for "Untitled-1" and "My Window" in the taskbar on "Hide" then "My Window" can be restored. It might happen again if Gorm is relaunched or "Document > New Application" is choosen again.
-> Once you "lost" "My Window" saving and reopening doesn't help, the window "My Window" remains lost, see attached "Test.zip" of a Gorm "Test.gorm" broken this way.
-> That behaviour seems to go away once you install SystemPreferences from SVN and toggle back and forth between the "GNUstep theme" and the "WinUXTheme" several times. It seems to happen far less frequently when the "GNUstep" theme is active.

Test.gorm.zip

https://savannah.gnu.org/bugs/?29762