First, I would strongly recommend against masking a specific element like you are doing. Masking is typically done a the window/dialog/grid level. I understand what you're trying to accomplish here but it looks a little ridiculous. Consider disabling your combo box and setting the text to "Loading..." instead.
That said, since FieldLabel extends Component, it has the various mask methods and you can call on your FieldLabel instance instead of the widget. This probably won't yield more desirable behavior and if not, it may be because the layouts/margins are a bit horked and the way to test this would be to try and add a mask on your User Details "window."
Finally, what version of GXT are you using? There is a bug in the mask method that wasn't fixed after GC and is NOT in the GPL version.