It has something to do with the following calculation at juce_DropShadowEffect.cpp:139:
const float radiusInset = (radius + 1) / 2.0f;
const float expandedRadius = radius + radiusInset;
If the resulting expandedRadius*nativeScaleFactor is not an integral number (where nativeScaleFactor is the scale factor of your monitor - so 2 for retina) then the gap seems to appear.
It’s odd because the drawShadowSection class use removeFromLeft/removeFromRight which shouldn’t result in there being a gap.