It should work on all windows..
The problem with the group,player and target windows is that gauges include the right click popup fuction.. So right clicking on a gauge or gauge text (player/target name) will bring up the menu.. You can stop that by putting all the gauge <pieces> above the nullbutton piece..
<Pieces>GW_Gauge1</Pieces>
<Pieces>GW_Gauge2</Pieces>
<Pieces>GW_Gauge3</Pieces>
<Pieces>GW_Gauge4</Pieces>
<Pieces>GW_Gauge5</Pieces>
<Pieces>GW_NullButton</Pieces>
But, then you won't be able to click on gauges to attain a target...
for windows that strecth You can change the button to something like this:
<Button item = "GW_NullButton">
<RelativePosition>true</RelativePosition>
<AutoStretch>true</AutoStretch>
<LeftAnchorOffset>0</LeftAnchorOffset>
<TopAnchorOffset>0</TopAnchorOffset>
<RightAnchorOffset>0</RightAnchorOffset>
<BottomAnchorOffset>0</BottomAnchorOffset>
<RightAnchorToLeft>false</RightAnchorToLeft>
<BottomAnchorToTop>false</BottomAnchorToTop>
</Button>
BTW.. Now that I think of it and of what caleal said.. You can get away with one NullButton in windows with sub_windows by putting all the subwindow pieces above the null button piece..
EDIT: I tested the above stretchy button and you need 2 more lines in there.. I added them to the bottom <RightAnchorToLeft> & <BottomAnchorToTop>
EDIT2: That is probably a better button template to use for all windows, even non-sizable windows... Because if resize the window in the xml.. You don't have to resize the button in the xml.. The game will do it for you