Note to self (and others so afflicted)—
Starting with this seemed to fix it:
set_fill_color(1.0,1.0,1.0)
fill_rect(0,0,scrSize[0],scrSize[1])

where i've done this at global scope:
scrSize=(1136,640) # iPhone 5
set_size(*scrSize) # min 16, max 2048

(my canvas is meant for landscape viewing)