Internal frames

[Pichler]

I try to build a simple text editor for multiple documents, based on internal frames and text areas in scroll panes. If I open one or more windows everthing is OK, as long as I do not activate other windows as the last opened. When I do so, I loose the cursor and can nothing mark any more in the text area. When I close all windows but one, then for this last one everthing is OK again. Has someone an idea what I do wrong?
Many thanks in advance
rudi