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The Default UI is in the game regardless of what you do. It seems to be the internal "if nothing is changed with X file, then use X file" setup.
Copper Enhanced takes the directory/file structure of the Default UI (i.e. the Textures folder will hold all of the appropriate texture files, Source for the XML and LUA, and so on... we'll leave out the Mods folder as it is very extra and NOT part of the Default UI), HOWEVER, if a file is NOT altered, that file isn't in the Modded UI at all. For example, if I have a Container Gump and a Hotbar and I wish to move the buttons or the scrollbar on the container gbump, I do so in the ContainerWindow.XML file; that file is then saved into the Copper Enhanced Source folder accordingly where it overrides the Default file: if I do NOTHING to the hotbar placement of the textures (i.e. don't change the XML), then I leave the hotbar.xml file out completely.
So what usually happens with a patch and the UI is that if you have a modded UI file that is changed in the patch, then you have to make the changes to your modded file to update the UI.
Case in point, Locking Hotbars.
In C-E, I have modded the hotbarsystem.LUA file. Because of this, people running C-E don't have the option to Lock Hotbar when they use the context menu. This afternoon, I went into the default hotbarsystem.LUA file and found the lines of code I had to copy/paste over and save into the C-E version of the hotbarsystem.LUA file to make the option work. Other mods who DON'T use that LUA file would be "automatically updated" as it would use the default codebase.
The problem I'm having is that C-E did NOT use the files Axel posted had the change and when I added them in, did not fix the problem, so I'm not exactly sure what's going on or where to look at the moment.