Bitmap skin Progressbar stretch issue

[T010]

I have this issue where the image used for the progressbar track is being stretched (for 125% Windows scale but issue also occurs on 150% scale) in an undesireable way.
Now I have to admit I do not understand the way the stretchY1 and stretchX1 parameters work, but both are set to value 7 (copied from existing bitmap skin, using same image dimensions). Changing this value does not seem to have any effect for me.

I do provide 100% 150% and 200% variants of the progressbar track image.

Top is the erroneous result, bottom is what is desired:

[Steven]

stretchY1 and stretchY2 are the vertical points where no stretching is done when upscaling the progress bar for higher dpi resolutions
eg.
height=20
stretchY1=2 means the top 2 pixels are not stretched
stretchY2=18 means the bottom 2 pixels are not stretched
the 16 pixels between are scaled in order to get the right overall scaled height for the progress bar
i dont think stretchX1 is doing anything

[T010]

I understand the theory of your explanation, it just does not seem to have any effect whatever value I insert.
However I have solved my above issue by adding some transparency to the progressbar image, in some magical way it then renders closer to the desired design.