I have a suggestion of how to implement it in the GUI:
A) If one moves the crop frame normally, MCU lines are ignored.
B) If you move the crop frame with the ALT key pressed, the crop frame always snaps to the nearest MCU block line/edge.
Will you implement this? Would be great!
The lossless crop process as I imagine it, in detail:
1) Already the initial point shall be set directly at a MCU edge point. To achieve this already within the first step, the user holds down the ALT key and moves the mouse around without a mouse-button pressed until the cursor points at the desired snapping point. Note: The mouse movements do not drag a frame yet, they only set the initial point!
2) Then the user press-and-holds a mouse-button and remains holding the ALT key and moves the cursor to determine the second edge point and hence already determines the complete crop frame.
3) Possibly continue to move the other edges/lines until the cropping frame is completed as desired.
4) Possibly move the whole frame along the MCU-raster by holding the ALT key and click-and-holding the mouse cursor within the frame square, followed by moving the mouse and hence the frame around, until the mouse button is released.
Note: If setting the initial edge on the raster (1) cannot be realised, it is also possible to simply start the initial edge on an arbitrary point (likely no MCU edge), set the other snapped edge(s)/line(s) first (2) and in the last step correct the initial arbitrary point to a MCU edge!