I tested it and it is upper left, not lower left.
This is the CanvasItem shader I used to test it:
shader_type canvas_item;
void fragment() {
if(distance(FRAGCOORD.xy, vec2(40, 40)) < 30.0)
COLOR = vec4(1,0,0,1);
else
COLOR = vec4(0,0,0,1);
}
(cherry picked from commit b71cec668a)
Samuel Ammonius
4b8fbb8939