Nice! This looks potentially useful and probably works fine for your stated purpose, I'd suggest a couple of tweaks...
Consider removing border around guide / selector lines do its only showing a single pixel line visually. Instead of making it a color, set the line to white and use an image effect that inverts whatever color it is over so its easy to see if you are moving the line over the edge of a border.
Consider allowing keyboard arrows to adjust seam/cuts if using with desktop. up and down arrow moves up and down one pixel, maybe shift up and down arrow jumps 10 pixels at a time. In seam mode I'm not sure what good keyboard shortcuts there would be, maybe tab to cycle between the two halves and up and down or left and right to drag chunks. maybe shift drags faster.
Also consider changing "Apply & New Seam" button to not be a hollow / disabled ghost button but style it to look like a normal button once there is an adjustment made so its obvious its now an option. It's not clear the button is clickable when in cut mode because it still looks disabled.
Allow manual dragging if mouse is within 5 - 10 pixels of any point of the guide/seam? Not sure what good range is... alsol change cursor to up and down resizer or left/right resizer cursor depending on vertical or horizontal seam mode.
(I often play cutting out chunks of images to save for art reference and being able to see pixel perfect edge for slicing would be useful.)
I use a tool called Pixelmator Pro for this but yours fine for what you've outlined of course. It has guidelines you can drag from ruler overlay and it also has an export to web for slicing up images in a similar way to your tool.
Just for reference I included a couple screenshots for some retro game art from a game called Mushroom Hunt by Polyduck because I like how they draw using very low resolution text mode graphics.
Consider removing border around guide / selector lines do its only showing a single pixel line visually. Instead of making it a color, set the line to white and use an image effect that inverts whatever color it is over so its easy to see if you are moving the line over the edge of a border.
Consider allowing keyboard arrows to adjust seam/cuts if using with desktop. up and down arrow moves up and down one pixel, maybe shift up and down arrow jumps 10 pixels at a time. In seam mode I'm not sure what good keyboard shortcuts there would be, maybe tab to cycle between the two halves and up and down or left and right to drag chunks. maybe shift drags faster.
Also consider changing "Apply & New Seam" button to not be a hollow / disabled ghost button but style it to look like a normal button once there is an adjustment made so its obvious its now an option. It's not clear the button is clickable when in cut mode because it still looks disabled.
Allow manual dragging if mouse is within 5 - 10 pixels of any point of the guide/seam? Not sure what good range is... alsol change cursor to up and down resizer or left/right resizer cursor depending on vertical or horizontal seam mode.
(I often play cutting out chunks of images to save for art reference and being able to see pixel perfect edge for slicing would be useful.)
I use a tool called Pixelmator Pro for this but yours fine for what you've outlined of course. It has guidelines you can drag from ruler overlay and it also has an export to web for slicing up images in a similar way to your tool.
Just for reference I included a couple screenshots for some retro game art from a game called Mushroom Hunt by Polyduck because I like how they draw using very low resolution text mode graphics.
- https://imgur.com/a/y38tdD0 - https://imgur.com/a/I4YIAVC