ShiftLinesHorizontally not working as intended

[Selur]

A user got a clip where I wanted to shift some lines horizontally.
So I wrote the following script:

import vapoursynth as vs
core = vs.core

def ShiftLinesHorizontally(clip: vs.VideoNode, shift: int, ymin: int, ymax: int) -> vs.VideoNode:
    # Validate clip format and subsampling
    if clip.format.color_family != vs.YUV or clip.format.subsampling_w != 0 or clip.format.subsampling_h != 0:
        raise ValueError("ShiftLinesHorizontalRange: only YUV444 input is supported.")
    
    # Ensure ymin and ymax are within valid range
    if ymin < 0 or ymin >= clip.height:
        raise ValueError(f"ShiftLinesHorizontalRange: ymin ({ymin}) is out of range.")
    if ymax < ymin or ymax >= clip.height:
        raise ValueError(f"ShiftLinesHorizontalRange: ymax ({ymax}) is out of range.")
    
    # If no shift is needed, return original clip
    if shift == 0:
        return clip

    width = clip.width
    height = clip.height

    # Create shifted version of just the target lines
    mid = clip.std.CropAbs(width=width, height=ymax-ymin+1, left=0, top=ymin)
    black = core.std.BlankClip(mid, width=abs(shift), height=mid.height, color=[0, 128, 128])
    
    if shift > 0:
        shifted_mid = core.std.StackHorizontal([black, mid.std.CropRel(right=shift)])
    else:
        shifted_mid = core.std.StackHorizontal([mid.std.CropRel(left=-shift), black])
    
    shifted_mid = shifted_mid.resize.Point(width=width, height=mid.height)

    # Build the output clip by stacking:
    # 1. Lines above ymin (unchanged)
    # 2. Shifted lines (ymin to ymax)
    # 3. Lines below ymax (unchanged)
    parts = []
    
    if ymin > 0:
        parts.append(clip.std.CropAbs(width=width, height=ymin, left=0, top=0))
    
    parts.append(shifted_mid)
    
    if ymax < height - 1:
        parts.append(clip.std.CropAbs(width=width, height=height-ymax-1, left=0, top=ymax+1))
    
    return core.std.StackVertical(parts)

and called it using:

import misc
clip = misc.ShiftLinesHorizontally(clip, shift=-24, ymin=468, ymax=479)

But something is wrong.
The 12 line I wanted are shifted (hurray), but the 12 lines above them got shifted too, and I don't see where I got it wrong.


=> does anybody see where my mistake is?

Cu Selur