2nd source causes flow rendering edge case

[ndsvw]

Hello, thanks for the project...

I found a problem in the main branch source code.

This is my code (simplified):

source [10000] ppp [darkgreen]
source [200] a [darkgreen]

ppp [5000] eee [lightgreen]
eee [?] vv

eee [2500] uu [purple]
vv [?] xx [red]

xx [?] mm [red]
mm [1000] vv1

This is what is rendered:

Only when removing the 2nd (smaller) stream from the source or reducing the depth, it gets better.