Mask location for dialogs/windows appears to have style issues.
For my main page, I have a border layout and in the center I display a content panel with a grid and other components. The mask in this panel is displayed (correctly) centered vertically and horizontally in this panel.
However, if I display a dialog (window) and then call mask on a content panel in that dialog/window, the mask shows up in the upper left corner of the content panel.
Colin had me try calling/setting etElement().getStyle().setPosition(STATIC); but this did not appear to have any effect.