[plt-scheme] Setting focus-ring of text-field controls programmatically

I'm switching between 4 text-field% instances in a tab-panel% by showing
the activated one and hiding the others using change-children. Switching
works fine, but I can't figure out how to make the text-field or its
editor or its editor's canvas draw a blue focus-ring like it is done
automatically when you allow switching between simple controls using the
tab key. 

Is it possible at all to make text-fields draw a blue focus-ring on OS X
when they are activated programmatically and get the keyboard focus?
What settings control the drawing of focus-rings on OS X?

I've already tried lots of things like making the field's editor the
caret-owner, setting allow-tab-exit to #t in editor-canvas, using text%
instances instead of text-field%, changing border settings of text
%...those #"$§%§$ focus-rings just don't show up.

Any help would be greatly appreciated!

Best regards,

Erich