Keep original-compressed preview 'slider' thumb center

[Weav3r]

Hello Angus, awesome project you've got going here. Currently the original-compressed preview 'slider' thumb is not centered when the image is zoomed in. It'll be helpful if the thumb can be made visible at all times to make comparisons and adjustments easier when zoomed in.
Thanks and hope you find the time to implement it. No rush. Regards