Thank you for the kind words. 🙂
Consider the 3 editing modes:
1) Setup mode: moving a vertex doesn't affect the UVs. Ie, moving a vertex deforms the texture.
2) Edit mode, deform unchecked: moving a vertex changes the UVs but doesn't affect the setup pose vertex positions. Ie, the position within the image used for the vertex changes, but the setup mode vertex doesn't move. This also results in the texture deforming.
3) Edit mode, deform checked: moving a vertex changes both the UVs and the setup pose vertex positions. Usually the texture is not deformed by doing this. If the texture was already deformed, then moving a vertex in this mode changes the deform with adding new deformation.
With mode #3, the boundary lines stop moving the vertex when the UVs hit the edge of the texture. The UVs always have a rectangular boundary (as seen in mode #2), but when you are also moving the vertex then the boundary from the UVs gets mapped to match the vertex space. If the vertex has never been moved relative to other vertices (ie, the texture is not deformed), then the boundary is rectangular. If it has been deformed, then the UV boundaries won't be rectangular when mapped to the vertex space. The boundary gets stretched to match how moving the vertex has deformed the texture. Each vertex may have different boundaries.
It is pretty easy to implement moving a vertex and adjusting the UVs at the same time, the tricky part is computing the boundary so the vertex movement stops when the UVs hit the edge of the texture. If we didn't do that, you'd hit the UV boundary but the vertex would keep moving, effectively stretching the texture around that vertex. The whole point of mode #3 is to adjust your UVs and setup pose vertices without adding new deformation.
