how to disable slider in thumb view.
Just edit the toolbar and remove it.
If you haven't edited the toolbars before, it's explained here: https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Editing_the_Toolbar.htm
Can't permanently remove it, I've right clicked and unticked images and also in customize >toolbars I've unticked many still..
Look for "thumbnail slider" on the toolbar itself and remove that.