Hello,
I'm not sure that slider is the right definition for the controller so I attach the screenshot, but the problem is that in 2.6.12 the slider does not move when you drag it, although you can go forward and backward through the video. I had to get back to 2.6.10 where this problem does not occur.
Maybe look at the post just before!
"Temptative fix for seek bar not working on El Capitan"
Try that one
http://avidemux.org/nightly/osx_qt5/Avidemux2.6_r2016_04_01_68a8f4989e6.dmg
Warning, it has been build with El Capitan and some options might be missing
But wouldn't be better to stay at 2.6.10? Is it that different from 2.6.12?
9 months of bugfixes