Hi!
I have a problem in Blender that never occured to me before. I am working on some meshes that uses trim sheets so in my example all faces are mapped separately to match my textures. Everything is working fine until I use any tool that have “Correct UVs” enabled. For example if I want to add loop cut and slide it blender will automaticly merge my uv islands in places where loop cut was applied. Same with usind Slide vertex od Edge slide. But I turn off Correct UVs then everything is working as expected - it just adds another edge loop.
As You can imagine sliding edge loops with them sliding on uvs as well is pretty helpfull but this merging shenanigans is pretty bad and its kind off against how it should work in my opinion. But maybe there are some hidden options that I missed like auto merge by distance in correct UVs or something. I tried to find something about it here or other forums but nothing helpfull came up.
Hmm… i couldn’t recognize what settings you may have differently than i use here (or i didn’t get the point… 
):
Huh, that’s interesting 
You are seemingly using same settings and yet You have different outcome. Only thing that came to my mind is that Maybe Your islands are not exactly aligned one after another as they are in 3d mesh itself if You know what I mean… 
Because for sure one on the islands is rotated so mabe others are not in order like You would put numbers on every edge 1-4 so on uv view edges 1 and 1 would be next to each other, 2 to 2 etc.
Also I was on Blender 3.6.5, tried this in 3.3.12(fresh install no addons) and 4.0(also fresh) and it behave the same way
IDK… blender 3.6.5 on linux standard scene cube scaled ~8x on one dimesnion deleted side faces smart unwrap with “edge border”… …and… still the same…:
cutTheCube.blend (121.6 KB)
Yes, in Your example it works good. But you have quite big gap between uv shells. When working with trim sheets edges are very close to each other to minimize waste of space. I moved each uv shell to be closer, could You try with this setup as with those distances it is collapsing those shells where loop cut is used. Thanks for Your time 
cutTheCubeighterMapping.blend (124.8 KB)
Ahh… try this… select edge ring… and then subdivide…
Yep, that works as it is not using Correct UVs yet if I want to slide this newly created edge loop… It collapses those verts as sliding uses Correct UVs, You can disable it but whats the purpose then
One quasi workaround is to use Correct Face Attributes when moving vertices or edges. But you need to be careful with it because if you forgot about it and move something without looking at uv’s or having textured viewport you might be surprised in a bad way as it will move your entire UV’s based on tranforms made in viewport
Hmm… something came up into my head… why do you need this margins for trim sheets at all… oohh wait youhave normals maps… right… … and if you have “fullu extended” UVs first… do all your editing and scale the islands as a last step to get those margins… ?? ( I know… not perfect… but still maybe a possible workarround ? For now… ??)
Yeah, workarounds are my life at the moment 
And yeah I am managing it like You said + using Correct Face Attributes when moving vertices or edges. It works but still those are unnecessary steps and are slowing me down.
I reported issue directly to Blender Foundation and will see what they say I’ll keep You posted!
Ok, it seems that Correct UVs is using old behaviour where in fact it is merging nearby vertices… Using Correct Face Attributes have it as optional toggle and it should be optional as well when using Edge slide with Correct UVs on etc.
No idea if or when it will be changed tho.
