ADP wrote:

The coffin corner is that place in an aircraft's speed and altitude range
where low speed stall runs into high speed mach buffet.
It has little to do with VNE.

Yes, it has a lot to do with Vne. It is the place where the stall speed
runs into *something* limiting the high speed. The upper speed limit is
Vne by definition, whatever the cause of this limit may be. Note that
Vne may vary with altitude, temperature, whatever. This red line is just
a simplification.

The mach number matters because in real life, the coffin corner was
relevant to military aircraft only. Their Vne was typically set by mach
speed.

Stefan