I had seen that and read that page multiple times. To me the confusing part was
“For the 1D strip, add a new segment with the start pixel beyond the matrix…”
When you add a segment after having configured a matrix, there is no field for start pixel there are fields called start x and start y.
This tripped me up as I was looking for the field called start LED as seen in the screenshot.
It wasn’t immediately apparent that start x is what was being referred to as start pixel.
Maybe it’s just me, but I found it confusing.
I agree with you that some things in wled is not that clear and needs a bit of experimntion ( other areas are there also if you go deep into 2D or other complex areas ) , i worte that docment based on the talk here and discord around how to mix 2d and 1D but like you i found this by experimenting . What you could do is go here Aircoookie/WLED-Docs: Documentation for the WLED project (kno.wled.ge) (github.com) and then modify the md file in your cloned repo and then might be add more pics and then make a PR once you feel it is more clear