Is it a bug or a feature ? (tabbing out of fields)

["Maurizio Ferreira" <ferreira@split.it>]

(plt scheme vers 103p1)
I've seen that tabbing out of a text-field leaves a black border
around the text inserted into the field.
To see the effect you need a text-field with some data, 
then tab out of the field, press again the tab key
until the focus returns to the field, and tab out again.
You'll see a black rectangle surrounding the text.
Is it a bug or a feature ?

Regards
Maurizio