Still beware that on OSX, if you get two different child like one on top left and one on bottom right, then everything gets repainted because the redraw surface is a rectangle not a rectangle list.
Relevant for the above issue you mention, could be this commit: (also pasted here for future reference to anyone hitting the same issue with opaque components)
https://github.com/julianstorer/JUCE/commit/e0f5218121054172bf9df6474ae37c29e6d24e11
This is the outcome of another long discussion in this forum about it.
Notice that this may lead to a different set of performance issues, so it should be used with caution (hence the enablement with a macro)