You mean history stack copying in the darkroom with the filmstrip? Interesting if that’s in the dev builds.
But the filmstrip is definitely erratic, even in the stable release. It often jumps to a different place other than simply advancing to the next image. You also have to be careful where your mouse is hovering. I wish that feature could be turned off because I’ve more than once applied a style to the wrong image just because my cursor was resting on a different image in the filmstrip.