[PD] pd at startup creates 2 canvases, why?

[Ivica Ico Bukvic]

I guess this is primarily for devs:

The title says it all. When pd starts up, it does 2 instances of
canvas_new() calls. More interestingly, it does not do canvas_free for
those two instances when closing pd, suggesting this is a memory leak.
So, what gives? Why does it create 2 invisible canvases, what is their
function, and how do they differentiate from the regular canvases.

NB: Not sure if this makes any difference but this is pd-l2ork code
based on pd-extended...