Thanks for testing it out. I saw how you did that fancy snap selection on your entry, but the Icon feature is meant to capture images that don't conform to the grid. I wanted to make the tool as mouse-only as possible, but I can see the advantage in having a key that expands the selection to the nearest grid box.
Viewing post in Atlas Decomposer jam comments
not sure if were asking how to do the "fancy" snap but if you're using (positive) integers only (ie, Screen coords), then ThisValue = (ThisValue/Size)*Size will return the nearest Snap position.
Since GetPointerX/Y() return Floats, ThisValue = FLOOR(GetPointerX()/Size)*Size will do the same while accounting for negative values (imagine a map editor using SetViewOffset() or the like where you may want Sprites, Text, etc, to Snap to position on both sides of 0,0).
for Atlas Decomposer, perhaps offering the option to Snap would be fitting. something like If ShiftKey() then Snap() else Dont while defining the Icon
I do use SetViewOffset() to keep the spritesheet in the upper left corner. The Shift option is a good idea. Right now Shift+drag, Ctrl+drag, and Alt+drag all do the same thing, but I'll think about some options to allow snapping to the underlying grid. Maybe Space to snap all current sides to the grid, and Shift+drag to snap while resizing.