The trick used on old macs was pretty good: instead of a solid triangle (which can mean "pyramid" or "arrow"), use increasingly narrow or increasingly wide layers:
This means "sorted small to large":
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This means "sorted large to small":
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I think that would be the best solution indeed.
Would the ones in favor of "not changing" (Robert Pluim, Joost Kremers) be ok with such a change or do they still find it counter-intuitive?
I also agree that no matter what is chosen, it'd behave the same on all platforms.
Philippe