Humm if the child which gets repainted is set to opaque, it should work fine.
check the debugger to see where this gets called.
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.
Try with JUCE_ENABLE_REPAINT_DEBUGGING see what's going on.