Before adding that as a new insert you add a couple of other layers. and then fit the insert in place without any large adjustments more then a few pixels. I took an even larger backdrop made up at 3900x1024 and with the two new layers added inserted one of the other ones you had and only nudged one end a bit to blend that in before the entire mesh was resized for the alternate method. But you may still seeing the starring problem a bit.
The first email notification for one post ended up without any post on the first thread where it shows closed. I then simply searched the section to find the extension seen here. Once any thread grows too large that will happen.