Layercake: a moveable cut & paste tool with line loop drawing
The cut & paste for the contraption is here, along with any updates in the future:
https://codeberg.org/woodring/decker.layercake
The APNG doesn’t seem to loop, so reload the page if the image doesn’t seem to do anything:

WARNING: THERE IS NO UNDO
Controls/Hotkeys:
L: start loop/segment or end as loop
S: start loop/segment or end as segment
click: add a line segment in loop or segment mode
T: toggle solid/transparent/invert
W: wipe (clear)
H: halt (stop drawing segment)
R: reset (halt and set brush and pattern to 1)
C: copy from card canvas
V: paste to card canvas
U: merge up (copy from card canvas treating white as transparent)
D: merge down (paste to card canvas treating white as transparent)
9/0: decrement/increment pattern (open listener to see current pattern number
1/2: decrement/increment brush (open listener to see current pattern number)
Hint: to see the brush and pattern, without the listener
start a line loop or segment with L or S
stretch out your line
use 9/0 and 1/2 to cycle through the patterns and brushes
press H to halt without drawing anything
the pattern and brush remains what it was set to