i think this is a bug. placing some text boxes above layouts on a page, selecting text in it and accessing tabstop-bar in textbox will not work: cursor selects the object UNDER the text box.

placing all graphical elements in an own layer, locking it and placing the textbox in a new layer above it will work.

reproducable on g4 / 10.2.8 and g5 / 10.3.1, one system german, the other english.