Using the default 2 tabbed app: https://github.com/appcelerator/titanium_mobile_blackberry/blob/master/test/apps/native/tibbtest/Resources/tabgroup_test.js
The window with the white background in each tab should fill the entire tabgroup, as the initial layout state of Ti.UI.Window should be Ti.UI.FILL. Instead, it appears that the Window's size is only big enough for its content, similar to what you would expect to with Ti.UI.SIZE (see attached). Setting the height value of the Window manually has no effect using either Ti.UI.FILL or a static value like 300. Ti.UI.FILL should be the default behavior.