Transparent App Window (Windows 10)

Windows
1

Just noticed this little problem with my sticky notes utility I built in JUCE.  It's supposed to be a fillRoundedRectangle all the way around, but where it overlaps a white window, I get a yellow fill...

Am I playing my ducks wrong, or is this a bug!? :)

2

Ah ... no I think it's my eyes.  It's a problem with the shadow not the colour...

Is there any way of fixing that better than just slopping a few transparent grey pixels in myself?