The front-end UX for scrolling that bitmap was quite frankly horribly badly designed.
Myself, I just get so frustrated about the idiocy.
It's still there: https://www.reddit.com/r/place/
They have may have fixed one or two scrolling issues since, but the main issue is that if you
a) press LMB
b) move the mouse, and move outside of the pixelized area
c) release LMB
.. it does weird things.