See the screenshot. The whole groupbox title is striked out in the background.
However, it's very light (#eeeeef), so you may not see this. That's because I
made a part of this "line" red.

Don't know if this is a bug or a feature (I personally don't like it very much),
but as you (Olivier) said that you have a TFT where you cannot distinguish very
light colors from white I wrote this bugreport.

Reproducible: Always
Steps to Reproduce:
1. 
2. 
3.