Live feed from watching-grass-grow.com:
Snap! Build Your Own Blocks
Oh so your watching?
Full screen is not on for a reason.
Fullscreen and Stepping are on so the image actually updates.
Very clever idea 
But I'm not seeing any change in behaviour when I hold the mouse down - it still stays cycling in the repeat loop
Works when I change to using key instead of mouse
Did you try holding down?
Yes
..............................
Mouse down doesn’t work at all neither does holding.
Then don't use it.
Ok, then. Just telling you it doesn’t work didn’t mean to offend you.
0 Offence Taken. 
I don't see anything
I saw the 3rd blade from the bottom left move................
The "URL as costume" does not handle cors. Costumes created this way are not clickable, because of being tainted.
You should use my costume from URL block, which takes care of CORS issues:
Custom blocks
is that your home?
no
Hold the mouse don't work if you click on the sprite "viewer", try to resize the viewer image (50%): if you click on the image, it doesen't work but if you click on the white background, it's working ! (exit the repeat block)