Test case attached for Windows (12px as platform size).
The font size is redefined to be 13px (1.083em). A Textfield is added to the container. The font size should be 10px ( 0.769em;). We observe that the font size is much smaller. It has a font size applied to 1.0em, not to 1.083em.

This is as if the platform font size was in used.