This is fairly self explanatory. Have a setting in addition to "Sample all layers" that only samples the current layer and the layers below.
As an illustrator i have not encountered any instances where i would need to sample layers on top of a layer while painting wth the mixer brush and it causes very strange behavior when you try to paint under a layer with a linework or some atmospheric transparent mist or light shafts above what i'm painting. Not having this feature is disruptive to mine and many artists workflows because while i don't use lines in my work i do use them as a guide early in the painting process and being able to paint under them for initial blocking would be very useful but is currently not possible as it samples the linework above and smears it around.
Currently when i want to use this brush on a layer that's not my top layer i have to hunt around for other layers above it and turn them off if there's pixels on top of what i'm painting.
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