How it looks now under my Linux:

Here is the code for testing. I hope the window always looks good now.
Code:
Runtime Enumeration Window
#WINDOW_Form_Parent
EndEnumeration
Runtime Enumeration Gadget
#GADGET_Form_Parent_Select
#GADGET_Form_Parent_SelectItem
#GADGET_Form_Parent_Cancel
#GADGET_Form_Parent_OK
EndEnumeration
XML$ = "<?xml version='1.0' encoding='UTF-8'?>" +
"<dialogs>" +
" <window flags='#PB_Window_SystemMenu | #PB_Window_ScreenCentered' text='Übergeordnet' id='#WINDOW_Form_Parent'>" +
" <vbox spacing='10'>" +
" <empty/>" +
" <gridbox colspacing='15' rowspacing='15' columns='4'>" +
" <empty/>" +
" <text text='Übergeordnet' flags='#PB_Text_Right'/>" +
" <combobox height='22' width='250' id='#GADGET_Form_Parent_Select'/>" +
" <empty/>" +
" <empty/>" +
" <text text='Übergeordnetes Element' flags='#PB_Text_Right'/>" +
" <combobox height='22' width='250' id='#GADGET_Form_Parent_SelectItem'/>" +
" <empty/>" +
" </gridbox>" +
" <empty/>" +
" <hbox>" +
" <empty/>" +
" <button text='Abbruch' id='#GADGET_Form_Parent_Cancel'/>" +
" <button text='OK' id='#GADGET_Form_Parent_OK'/>" +
" <empty/>" +
" </hbox>" +
" <empty/>" +
" </vbox>" +
" </window>" +
"</dialogs>"
If ParseXML(0, XML$) And XMLStatus(0) = #PB_XML_Success
If CreateDialog(0) And OpenXMLDialog(0, 0, "")
For i = 0 To 2
AddGadgetItem(#GADGET_Form_Parent_Select, -1, "Select_" + i)
AddGadgetItem(#GADGET_Form_Parent_SelectItem, -1, "SelectItem_" + i)
Next
Repeat
Event = WaitWindowEvent()
Until Event = #PB_Event_CloseWindow
Else
Debug "Dialog error: " + DialogError(0)
EndIf
Else
Debug "XML error: " + XMLError(0) + " (Line: " + XMLErrorLine(0) + ")"
EndIf